Intrinsic viscosities of polystyrenes (PS), polymethyl meth-acrylates (PMMA), and their random copolymers have been determined at different temperatures to estimate chain flexibility of homopolymers and copolymers. The coefficients Ke, the chain flexibility descriptor, for PS decreases with increasing Mnand have positive values. For PMMA the Keincrease slightly with increasing Mnand have negative values. The Kevalues for S/MMA copolymers are all positive, showing that the chain flexibility in copolymers is predominantly influenced by styrene concentration. For PS the long-range parameter B and the excluded volume factor Z increase from 25 to 45°C and thereafter remain constant. The short-range parameter, A, for PS remains virtually unchanged with the change in temperature. For PMMA, the short-range, the long-range, and the excluded volume factors remain virtually constant between 25 and 65°C.