The specific interactions in ternary 4-hydroxybenzoic acid (HBA)/poly(2-vinylpyridine) (P2VPy)/poly(N-vinyl-2-pyrrolidone) (PVP) blends were studied by differential scanning calorimetry, Fourier transform infrared (FTIR) spectroscopy, and electron microscopy. FTIR study shows the existence of hydrogen-bonding interactions between HBA and PBVPy as well as PVP. The addition of a sufficiently large amount of HBA produces a blend showing one glass-transition temperature (T-g). Microscopic study shows a drastic reduction in domain size in single-T-g blends.