Fourier transform infra-red spectroscopy was used to study the specific interactions between three aliphatic dicarboxylic acids, namely succinic acid (SCA), suberic acid (SBA) and dodecanedioic acid (DDA), and two proton-accepting polymers, poly(2-vinylpyridine) (P2VPy) and poly(N-vinyl-2-pyrrolidone) (PVP). P2VPy interacts with the acids through hydrogen-bonding interaction as well as ionic interaction, while PVP interacts with the acids through hydrogen-bonding interaction. The intensity of interaction decreases in the order SCA > SBA > DDA, which is in the reverse order of the spacer length between two carboxylic groups of the acid. The miscibility of ternary P2VPy/PVP/acid blends was examined by differential scanning calorimetry, scanning electron microscopy and solid-state C-13 nuclear magnetic resonance spectroscopy. The interaction strength and the spacer length play important roles in the miscibility and phase behavior of P2VPy/PVP/acid blends. SCA interacts most strongly with P2VPy and PVP, but SBA has a better miscibilization effect due to its longer spacer. However, DDA, which has the longest spacer, is unable to miscibilize P2VPy/PVP blends.