The degree of cross-linking for linear poly(ethylenimine) hydrochloride, cross-linked using malonaldehyde generated in situ, was determined from the ratio of the cross-link to backbone hydrogens obtained using H-1 NMR spectroscopy. The a.c. conductivity is highest at intermediate degrees of cross-linking (ca. 0.45), approximately 1 x 10(-3) S/cm at room temperature and 75% relative humidity. IR and Raman spectroscopy were used to characterize the cross-linked network. The presence of the beta-amino-ethenyliminium cross-linker units can be identified through a series of bands between 1570 and 1640 cm(-1). Other changes in the spectra identified as a function of degree of cross-linking are discussed. (C) 2008 Elsevier Ltd. All rights reserved.