Blending of polymers is rather a new technique to get a material with tailored properties. However, these properties depend upon various factors like polymers used and their miscibility, the composition of the material, temperature etc. Further, the techniques available for testing of miscibility are not really reliable and face a huge critic. Therefore, the main purpose of this research work was to investigate the miscibility of polymers under various conditions by using the most popular techniques and to correlate with the mechanical properties of polystyrene (PS)/ poly (styrene-co-acrylonitrile) (PSAN) blend prepared by solution casting method. The product was an interesting as it would combine the superior processing characteristics of PS and the better weather UV resistant, excellent oil-resistant, and superior mechanical properties of PSAN. In this thesis an attempt was made to investigate thoroughly the effect of composition, temperature, and solvent on the miscibility, and hence on the morphology, thermal, and mechanical properties of the resulting blends. The miscibility of polystyrene (PS), and poly(styrene-co-acrylonitrile) (PSAN) blend in tetrahydrofuran (THF), dimethyl form amide (DMF), chloroform (CHCl3), benzene, and acetone was investigated by viscometric, density, and refractometric methods over an extended range of concentration, composition, and temperature. The miscibility behavior of different blend systems was examined on the basis of signs of various interaction parameters proposed by Chee (μ and ΔB), Garcia (Δ[η]m), Jiang and Han (β) and Sun (α). The viscometry results were then correlated with the miscibility findings for the same blend systems by refractive index and density techniques. On the basis of solution state xx studies we concluded the degree of miscibility as; PS/PSAN/DMF˃ PS/PSAN/CHCl3˃ PS/PS/PSAN/THF˃ PS/PSAN/Acetone˃ PS/PSAN/Benzene. Rheological measurements of PS/PSAN/THF blend system also indicated that the miscibility was dependent upon blend compositions. Various rheological parameters vs frequency plots indicated miscibility for composition, 50/50, and immiscibility for all other blend (30/70 and 70/30) compositions. Whereas, the plots of these rheological parameters vs weight % of PS in the blend showed negative deviation from the additivity line and hence immiscibility for all the blend compositions and similar observations were made in case of shear viscosity, and shear stress vs shear rate plots. The miscibility of the PS/PSAN blend films cast using different solvents (THF, DMF, and CHCl3) was also probed in the solid state with the help of scanning electron microscopy (SEM), Fourier transform infra-red (FTIR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetry (TG), and tensile testing. Morphological observations revealed partial miscibility for PS/PSAN blend films cast from THF and DMF, whereas immiscibility for the PS/PSAN/CHCl3 blend films. FTIR measurements indicated immiscibility for all the blend systems, irrespective of the type of casting solvent, as none of the spectra for different blend compositions displayed variations in peak shifting. Thermal (DSC, and TG) studies showed consistency with the morphological results, endorsing the partial miscibility of the PS/PSAN/THF, and PS/PSAN/DMF blends, and immiscibility of the PS/PSAN/CHCl3 blend. All the tensile properties of the PS/PSAN/THF blend system showed positive deviations from the rule of mixtures and were decreased with the increase in weight % of PS in the blend, indicating compatibility of the said blend system. The decrease in various tensile properties with the increase in PS contents of the blend was assigned to the inferior mechanical properties of PS along with the decrease in interfacial adhesion between the two phases. Maximum synergy was observed for PS/PSAN/THF blend, 25/75 composition, in all the tensile properties which were attributed to the intra-molecular repulsive effect, characteristic of the homopolymer/copolymer blend system, and some structural similarity between the blend components. The PS/PSAN/DMF blend system retained its partial miscibility in tensile properties by showing positive deviation for 25/75 and negative deviations from the additivity line for the remaining blend, 50/50, and 75/25 compositions. The PS/PSAN/CHCl3 also retained its immiscible and incompatible nature by displaying negative deviations from the additivity line, for all the tensile properties of different blend compositions, and was increased with the increase in PS contents in the blend. The extent of miscibility of PS/PSAN blends characterized in the solid state was PS/PSAN/THF ˃ PS/PSAN/DMF ˃ PS/PSAN/CHCl3. Based over these results, THF was classified a better solvent for solution cast PS/PSAN blends than rest of the investigated solvents. Further, the blend composition had the impact over the miscibility and hence over the mechanical properties of the blend. The mechanical properties were improved with the increase in miscibility of the blend.