Complexes of polyvinyl pyrrolidinone-polyacrylic acid (PVP-PAA) were prepared by photopolymerisation from a mixture of the monomers NVP and AA. The complexes were characterised by means of differential scanning calorimetry, Fourier transform infrared spectroscopy (Ftir), potentiometric titration, swelling studies and gel permeation chromatography. The Ftir spectra of PVP-PAA copolymer complexes indicates hydrogen bonding between the carbonyl group in the PVP and the carboxylic acid group in the PAA moiety. As the percentage of AA increases in the copolymer there is evidence of increased intermolecular hydrogen bonding between the carboxylic acid groups of the AA segments. Swelling of the PVP-PAA complex in a higher pH medium is significantly different from results in low pH solutions. The critical pH range was found to be between 4.07 and 4.49. Above a pH of 4.49, there is a progressive break up of the polymer chain due to a reduction in the amount of intermolecular hydrogen bonding. There is also a significant increase in the solubility of the copolymer complex at higher pHs. The low solubility of the copolymer at low pH may make the complex suitable for gastric drug delivery systems. (C) 2003 Elsevier Ltd. All rights reserved.