Poly(N-acryloyl-N' -methylpiperazine) (PAMP) forms complexes with four strong acidic polymers, namely, poly(styrenesulfonic acid), poly(vinylphosphonic acid), poly(acrylic acid) and poly(methacrylic acid) in ethanol/water (1:1) solution. The nature of interpolymer interactions in various complexes was studied by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Both the carbonyl oxygen and the amide nitrogen of PAMP are involved in hydrogen-bonding interactions. Some of the amine nitrogens of PAMP are protonated and therefore PAMP also interacts with the acidic polymers through ionic interactions.