The effects of acetic acid on the polymerization characteristics of nylon-6 are investigated in a reactor model that consists of a continuous flow stirred tank reactor (CSTR) and a tubular reactor connected in series. Mathematical models for the CSTR and the tubular reactor have been established and solved by numerical methods. In the CSTR, the monomer conversion and the molecular weights are increased as the feed acetic acid concentration is increased. In the tubular reactor, the acid acts as both a catalyst and a modifier for the polymerization reaction. The effects of the feed acetic acid content on the zeroth, first and second moments and the polydispersity index of the polymer have been discussed.