This contribution reports on the synthesis of hyperbranched polylysines via thermal polymerization of L-lysine hydrochloride. Polymerization of L-lysine hydrochloride in the presence of one equivalent KOH at 150°C resulted in polymers with a number-average molecular weight of 4600 g/mol and a polydispersity of 2.6 after 48 h. The rate of polymerization could be significantly enhanced and the polymer molecular weight improved by carrying out the polymerization with 3 mol % of an amidation catalyst. Among the different catalysts that were investigated Zr(OnBu)4 was found to be the most effective. Unequivocal support for the branched architecture of the polymers was obtained from 1H NMR spectroscopy, which allowed the identification and quantification of the four different structural units that constitute the polymer, viz. Nα and Nε linked linear units, dendritic units and terminal units. The structure of the polymers was found to be relatively independent of the reaction conditions. The degree of branching and the average number of branches varied between 0.35-0.45 and 0.15-0.25, respectively. © 2007 Wiley Periodicals, Inc.