Polyamideamine-epichlorohydrin (PAE) resin is a crosslinked heteropolymer having cationic charges, and has been widely used as a wet strength agent of paper in the papermaking process. In this study, more accurate molecular mass values of PAE were determined by size exclusion chromatography attached. with a multi angle laser light scattering detector (SEC-MALS). The obtained weight average molecular mass (M-w) values of commercial and laboratory-made PAE samples were 1,140,000 and 494,000, respectively, and these values were much higher than those reported so far. SEC-MALS analysis also revealed that PAE molecules had highly crosslinked structures and extremely wide molecular mass distributions. Molecular mass values and content of 3-hydroxy-azetidinium (AZR) groups in PAE and PAE intermediates during the course of PAE synthesis were also determined by the SEC-MALS and H-1 NMR methods, and the relationships between these values and the wet strength development of handsheets prepared with these PAE samples were studied. (c) 2005 Wiley Periodicals, Inc.