The present investigation is based on the study of the evaluation of the following factors on the photolysis of riboflavin (RF) in aqueous/organic solvents. 1. Solvent Effect on the Photolysis of RF The kinetics of photolysis of RF in water (pH 7.0) and in organic solvents (acetonitrile, methanol, ethanol, 1-propanol, 1-butanol, ethyl acetate) has been studied using a multicomponent spectrometric method for the assay of RF and its major photoproducts, formylmethylflavin and lumichrome. The apparent first-order rate constants (kobs) for the reactions range from 3.19 (ethyl acetate) to 4.61×10−3 min−1 (water). The values of kobs have been found to be a linear function of solvent dielectric constant implying the participation of a dipolar intermediate along the reaction pathway. The degradation of this intermediate is enhanced by the polarity of the medium. This indicates a greater stabilization of the excited-triplet state of RF with an increase in solvent polarity to facilitate its photoreduction. The rate constants for the reaction show a linear relation with the solvent acceptor number showing the magnitude of solute–solvent interaction in different solvents. It would depend on the electron–donating capacity of the RF molecule in organic solvents. The values of kobs are inversely proportional to the viscosity of the medium as a result of diffusion-controlled processes. 2. Ionic Strength Effects on the Photodegradation Reactions of RF It involves the study of the effect of ionic strength on the photodegradation reactions (photoreduction and photoaddition) of RF in phosphate buffer (pH 7.0) using the specific multicomponent spectrometric method mentioned above. The rates of photodegradation reactions of RF have been found to be dependent upon the ionic strength of the solutions at different buffer concentrations. The values of kobs for the photodegradation of RF at ionic strengths of 0.1–0.5 M (0.5 M phosphate) lie in the range of 7.35–30.32 × 10−3 min−1. Under these conditions, the rate constants for the formation of the major products of RF, lumichrome (LC) by photoreduction pathway, and cyclodehydroriboflavin (CDRF) by photoaddition pathway, are in the range of 3.80– 16.03 and 1.70–6.07 × 10−3 min−1, respectively. A linear relationship has been observed between log kobs and √μ/1+√μ. A similar plot of log k/ko against √μ yields a straight line with a value of ~+1 for ZAZB indicating the involvement of a charged species in the rate determining step. NaCl promotes the photodegradation reactions of RF probably by an excited state interaction. The implications of ionic strength on RF photodegradation by different pathways and flavin–protein interactions have been discussed. 3. Metal Ion Mediated Photolysis of RF The effect of metal ion complexation on the photolysis of RF using various metal ions (Ag+, Ni2+, Co2+, Fe2+, Ca2+, Cd2+, Cu2+, Mn2+, Pb2+, Mg2+, Zn2+, Fe3+) has been studied. Ultraviolet and visible spectral and fluorimetric evidence has been obtained to confirm the formation of metal-RF complexes. The kinetics of photolysis of RF in metal- RF complexes at pH 7.0 has been evaluated and the values of kobs for the photolysis of RF and the formation of LC and LF (0.001 M phosphate buffer) and LC, LF and CDRF (0.2–0.4 M phosphate buffer) have been determined. These values indicate that the rate of photolysis of RF is promoted by divalent and trivalent metal ions. The second-order rate constants (k ′ ) for the interaction of metal ions with RF are in the order: Zn2+ > Mg2+> Pb2+ > Mn2+ > Cu2+ > Cd2+ > Fe2+ > Ca2+ > Fe3+> Co2+ > Ni2+ > Ag+. In phosphate buffer (0.2-0.4 M), an increase in metal ion concentration leads to a decrease in the formation of LC compared to that of CDRF by different pathways. The values of kobs for the photolysis of RF have been found to increase with a decrease in fluorescence intensity of RF. The photoproducts of RF formed by pathways have been identified and the mode of photolysis of RF in metal-RF complexes has been discussed. 4. Preparation, Characterization and Formation Kinetics of RF-Ag NPs Riboflavin conjugated silver nanoparticles (RF–Ag NPs) have been prepared by photoreduction of Ag+ ions and characterized by UV–visible spectrometry, spectrofluorimetry, dynamic light scattering (DLS), atomic force microscopy (AFM) and FTIR spectrometry . These NPs exhibit a surface plasmon resonance (SPR) band at 422 nm due to the interaction of RF and Ag+ ions. The fluorescence of RF is quenched by Ag NPs and the total loss of fluorescence is due to complete conversion of RF to RF–Ag NPs conjugates. FTIR studies indicate the appearance of an intense absorption peak at 2920 cm–1 due to the interaction of RF and Ag. DLS has shown the hydrodynamic radii (Hd) of RF–Ag NPs in the range of 57.9–72.2 nm with polydispersity index of 27.5–29.0 %. AFM indicates that the NPs are spherical in nature and polydispersed with a diameter ranging from 57 to 73 nm. The effect of pH, ionic strength and reducing agents on the particle size of NPs has been studied. At acidic pH (2.0–6.2) aggregation of RF–Ag NPs occurs due to an increase in the ionic strength of the medium. The rates of formation of RF–Ag NPs on UV and visible light irradiation have been determined in the pH range of 8.0–10.5 and at different concentration of Ag+ ions. The photochemical formation of RF– Ag NPs follows a biphasic first–order reaction probably due to the formation of Ag NPs in the first phase (fast) and the adsorption of RF on Ag NPs in the second phase (slow).