Both poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) form complexes with poly(4-vinylpyridine) (P4VPy) and paly(2-vinylpyridine) (P2VPy) in ethanol/water (1:1) solution. All the complexes show a positive deviation in their glass transition temperature values. The nature of interpolymer interactions in various complexes was studied by Fourier transform infrared (FTi.r.) spectroscopy and X-ray photoelectron spectroscopy (XPS). PMAA interacts with P4VPy and P2VPy via hydrogen-bonding interactions. However, both FTi.r. and XPS provide evidence of ionic interactions in the PAA complexes. The different complexation behaviour between PMAA and PAA is attributed to the higher acidity of PAA than that of PMAA.