Hydrogen bonding formation in a polymer complex between poly(4-vinylpyridine) (P4VPy) and maleic acid-alt-ethylene (MA-alt-E) copolymer has been studied. Fourier transform infrared spectroscopic (FT-IR) studies have evidenced that the carboxylic groups of MA-alt-E interact with the pyridine ring of P4VPy. The existence of hydrogen bonding interactions in these systems is shown by the appearance of new bands at 1640 and 1503 cm(-1) and a broadening of the -C=C- band of the pyridinic ring. The thermodynamic analysis of the hydrogen bonding was performed through the estimation of the enthalpy of mixing in the polycomplex formation process. DeltaH(M) was determined by means of an association model developed by Painter and co-workers and by flow calorimetric direct measurements. These results showed a high negative value for the enthalpy of mixing, which is consistent with the blend miscibility given that DeltaH(M) is the main contribution to the change of the free energy of mixing, AGM. (C) 2003 Elsevier Science B.V. All rights reserved.