The miscibility or complexation of poly(styrene-co-acrylic acid) containing 27 mol % of acrylic acid (SAA-27) and poly(styrene-co-N,N-dimethylacrylamide) containing 17 or 32 mol % of N,N-dimethylacrylamide (SAD-17, SAD-32) or poly(N,N-dimethylacrylamide) (PDMA) were investigated by different techniques. The differential scanning calorimetry (DSC) analysis showed that a single glass-transition temperature was observed for all the mixtures prepared from tetrahydrofuran (THF) or butan-2-one. This is an evidence of their miscibility or complexation over the entire composition range. As the content of the basic constituent increases as within SAA-27/SAD-32 and SAA-27/PDMA, higher number of specific interpolymer interactins occurred and led to the formation of interpolymer complexes in butan-2-one. The qualitative Fourier transform infrared (FTIR) spectroscopy study carried out for SAA-27/SAD-17 blends revealed that hydrogen bonding occurred between the hydroxyl groups of SAA-27 and the carbonyl amide of SAD-17. Quantitative analysis carried out in the 160-210 degrees C temperature range for the SAA-27 copolymer and its blends of different ratios using the Painter-Coleman association model led to the estimation of the equilibrium constants K-2, K-A and the enthalpies of hydrogen bond formation. These blends are miscible even at 180 degrees C as confirmed from the negative values of the total free energy of mixing Delta G(M) over the entire blend composition. (c) 2006 Wiley Periodicals, Inc.