Poly(N-aclyloyl-N＇-phenylpiperazine) (PAPP) forms miscible blends with poly(p-vinylphenol) (PVPh), and it forms complexes with three stronger acidic polymers, namely, poly(styrenesulfonic acid) (PSSA), poly(vinylphosphonic acid) (PVPA), and poly(acrylic acid) (PAA). Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were used to study the nature of interpolymer interactions in these blends and complexes. All three types of interacting sites of PAPP are involved in interactions with the acidic polymers. PAPP interacts with PVPh and PAA through hydrogen-bonding interaction, whereas it interacts with PSSA and PVPA through a combination of hydrogen-bonding and ionic interactions. As compared to poly(N-aclyloyl-N＇-methylpiperazine), the interactions between PAPP and the acidic polymers are less intense because of the steric shielding effect of the phenyl groups and the lower basicity of the tertiary amine nitrogen.