Poly(vinylphosphonic acid) (PVPA) forms interpolymer complexes with poly(4-vinylpyridine) and with poly(2-vinylpyridine) in ethanol/water (1/1) solution. Both the yield and the composition of the complex depend on the feed composition. All the complexes do not show distinct glass transition temperatures (T(g)s) up to the onset of degradation. It is conceivable that the strong interactions between PVPA and poly(vinylpyridine)s might raise the T(g)s above the degradation temperatures. An X-ray photoelectron spectroscopic study shows that some of the pyridine groups in various complexes are protonated as shown by the existence of two nitrogen environments in the N1s spectra. The extent of protonation of pyridine increases with increasing PVPA content in the complex. The P2p spectra show the existence of two phosphorus environments, one arising from phosphonic acid and the other from phosphonate. Therefore, ionic interactions between pyridinium and phosphonate ions are responsible for the complexation.