Anionic ring-opening polymerization of lactams leads to the formation of poly(lactams) or polyamides. This work aimed at comparing the performance of four methods for measuring the conversion of epsilon-caprolactam (CL) to polyamide 6. The latter was either a homopolymer (PA6) or grafts onto polystyrene (PS-g-PA6 graft copolymer). Those four methods were mass balance based on solvent extraction (methanol, water, THF, or acetone), mass balance based on vacuum drying at 140 degrees C, thermogravimetric analysis (TGA), and elemental analysis based on nitrogen. The mass balances based on methanol extraction and vacuum drying at 140 degrees C and TGA were all suitable for measuring the conversion of CL, whether the resulting polymer was the PA6 or PS-g-PA6. The mass balance based on water extraction was good for the PA6 and not good for the PS-g-PA6. The elemental analysis based on nitrogen was not suitable for the PA6 nor for the PS-g-PA6. (c) 2006 Wiley Periodicals, Inc.