The hypochromism of styrene-methyl methacrylate random copolymers at 269.5 nm is attributed to strong interactions between the carbonyl groups of the ester and the phenyl rings. Support to this correlation is provided by model studies. Namely, the effect of several esters and other carbonylic solvents on the extinction coefficients of ethylbenzene and atactic polystyrene confirms the nature of the hypochromism. In order to eliminate the hypochromism of S-MMA copolymers, ester-type solvents are proposed. Indeed, when such solvents are used, a linear correlation between optical density at 269.5 nm and polystyrene content in the copolymers is found. Implications in gel permeation chromatography analysis are discussed.