Sulfuric acid solutions of copolyarylene phthalides of the diphenyloxide series and their copolymers with methyl methacrylate and styrene were studied by the photometric method. According to spectral data, regardless of the ratio of single (α) and double (β + γ) phthalide groups in the macrochains of copolyarylene phthalides in their copolymers with methyl methacrylate, there are ~20–26 mol of MMA per 1 mol of phthalide units. The structure of copolyarylene phthalides is a determining factor in the formation of copolymers with styrene, in which either styrene units or phthalide fragments predominate (~15–40 and ~2–5 mol of styrene per mole of phthalide unit).