The effects of carboxylic acid on the polymerization characteristics of nylon-6 were investigated in reactor models that consist of a continuous-flow stirred tank reactor (CSTR) and a tubular reactor with a diffusional water-removal system, which are connected in series. Mathematical models for the CSTR and the tubular reactor were established and solved by numerical methods. In the CSTR, with an increase of the feed acetic acid content, the monomer conversion, and the molecular weights are increased. In the tubular reactor, the acid behaves like a catalyst and a modifier at the same time in the polymerization of nylon 6. The effects of the feed acetic acid content and the diffusional water removal on the zeroth, first, and second moments and the polydispersity index of the polymer were investigated. The polydispersity index is greatly affected by the feed content of carboxylic acid in the CSTR, but it finally approaches to values of similar to 2 in the tubular reactor. The diffusional water removal is found to have little effect on the polydispersity index of the polymer.