The random copolycondensation of isoplithalic acid/terephthalic acid with various combinations of bisphenols (M-1 and M-2) with a tosyl chloride/dimethylformamide/pyridine condensing agent was carried out to investigate the effects of the monomer reactivity ratios, r(1)' and r(2)' on the reaction, like r(1) and r(2) in radical copolymerization. The ratios were calculated from the probabilities of finding an M-2 unit next to an M-1 unit and of finding an M-1 unit next to an M-2 unit, which were determined by an NMR analysis of the resultant copolymers. They were discussed with respect to the inherent viscosities (molecular weights) of the resultant copolymers. There was a fairly good relationship between r(1)' and r(2)' and the inherent viscosity values of the copolymers, indicating that copolycondensation could be facilitated by a combination of bisphenols; the lowering of r(1)' and r(2)' was indicative of random distributions in the copolymers. (C) 2003 Wiley Periodicals, Inc.