The present work involves a study of the photolysis of riboflavin (vitamin B2) (RF), norfloxacin (antibiotic) (NF) and cyanocobalamin (vitamin B12) (B12) in various liposomal preparations having a composition of cholesterol (CH) and phosphatidylcholine (PC) with a ratio of CH:PC (13.50: 10.80–16.20 mM) and the evaluation of the effect of compositional variations on the kinetics of degradation and photostabilization of these drugs. UV and visible spectroscopy, fluorimetry, dynamic light scattering (DLS), atomic force microscopy (AFM) and high performance liquid chromatography (HPLC) have been employed to investigate various aspects of this work. The literature on the analytical methods used in the study of liposomes, formulation of liposomes and stabilization of liposomal drugs has extensively been reviewed. The observations on the study of the individual drugs used are as follows. Riboflavin (RF) The apparent first–order rate constants (kobs) for the photolysis of RF in liposomal preparations lie in the range of 1.73–2.29×10–3 min–1 compared with a value of 8.08 × 10–3 min–1 for the photolysis of RF in aqueous solution (pH 7.4). The values of kobs decrease linearly with an increase in PC concentration in the range of 12.12–14.85 mM, indicating the stabilization effect of PC on RF with a stabilization ratio of around 4. This is confirmed by a loss of fluorescence intensity of RF with an increase in PC content as a result of the quenching of excited singlet state and the formation of a charge–transfer complex between PC and RF. The values of kobs decrease with an increase in PC concentration indicating the effect of PC on RF stabilization. The entrapment efficiency of RF in liposomes has been determined as 25.9–42.3%. The assay of RF and its photoproduct, lumichrome (LC), in liposomal preparations has been carried out by a two– ii component spectrometric assay at 445 and 356 nm with a correction for linear irrelevant absorption to eliminate interference from liposomal components. The study indicates that RF can be stabilized against light by a modification of the liposomal formulation with an increase in PC concentration. The mechanism of RF and PC interaction has been discussed. Norfloxacin (NF) The values of kobs for the photolysis of NF in liposomal preparations are in the range of 1.05–2.40×10–3 min–1 and depend on the concentration of PC in liposomes. The value of kobs for the photolysis of NF in aqueous solution (pH 7.4) is 8.13×10–3 min–1 indicating a stabilization ratio of 3–7 in various liposomal preparations. A linear relation between kobs and PC concentration with a negative slope has been observed to show that PC exerts a stabilizing effect on NF in liposomes. DLS has indicated an increase in the size of NF encapsulated liposomes with an increase in PC concentration. Similar to RF the quenching of excited singlet state of NF by PC indicated by a loss of fluorescence suggests an interaction between NF and PC to form a charge–transfer complex. It results in the reduction of NF to form [NF–] species which cause stabilization of NF in liposomal preparations. The entrapment efficiency of NF in liposomal preparations amounts to 41–56%. The mode of interaction of NF and PC is similar to that of RF and PC to cause stabilization of NF in liposomal preparations. Cyanocobalamin (B12) B12 is a relatively large molecule compared to RF and NF and its photochemical behavior in liposomes has also been studied. The values of kobs for the photolysis of B12 in liposomes have been found to be in the range 0.52–2.24 ×10–3 min–1, compared to that of 3.21×10–3 min–1 for B12 in aqueous solution (pH 5.0) and an entrapment efficiency in the range of 26.4–38.8%. In this case also a linear relation has been observed between the values of kobs and PC concentration with iii a negative slope indicating the influence of PC in inhibiting the rate of photolysis of B12. This also appears to be due to the involvement of a charge–transfer complex between B12 and PC that results in the stabilization of vitamin B12. The stabilization ratio of B12 in liposomal preparations has been determined as 1–6. The present study on the photolysis of RF, NF and B12 suggests that these drugs may be stabilized in liposomal preparations. The mode of stabilization involves the participation of a charge–transfer complex to cause the reduction of the drug to the species that have low susceptibility to photolysis. The values of the second–order rate constants for the photochemical interaction of RF, B12 and NF with PC are 1.48, 0.32 and 8.92×10–2 M–1 min–1, respectively, suggesting that PC exerts the greatest effect on the stabilization of RF, followed by those of B12 and NF. This could be due to the relative ease of electron donation from PC to the drug and charge–transfer complex formation between them. This would probably depend on the redox potentials of these drugs under the reaction conditions involved. On the basis of the results obtained in this study it may be suggested that such an approach could be useful in the stabilization of photoliable drugs in liposomal preparations.