Free radical polymerization kinetics of N-vinylformamide was investigated in bulk and in aqueous solution. The molecular weight development with N-vinylformamide conversions was measured using a gel permeation chromatography (GPC). The bulk process showed a typical "gel effect" from the beginning of the polymerization. The molecular weight increased steadily at low conversion and leveled off at high conversion. In the solution polymerization, the "gel effect" became less pronounce with decreasing monomer concentration. When the monomer concentration was less than 40 wt%, no auto-acceleration was observed from the time-conversion curves. The k(p)/k(t)(1/2) values of the polymerization system were determined using the initial rates of bulk polymerization and the number average molecular weight data at 50, 60 and 70 Z degreesC. A kinetic model based on free volume theory was proposed to describe the polymerization kinetics and molecular weight development. The model agreed with the time-conversion and conversion-average molecular weight data. (C) 2001 Elsevier Science Ltd. All rights reserved.