The interpolymer specific interaction of proton donating polymer (PDP) and proton accepting polymer (PAP) in toluene was studied by viscometry coupled with light scattering. The viscometric experiment results show that the stronger the interpolymer interaction is, the higher the viscosity of PDP/PAP blend solution than the weight-average of both components at high concentration, in contrast to lower viscosity at low concentration. Based on the relationship of viscosity enhancement factor with polymer level in solution, a new polymer-polymer interaction parameter k(a) to estimate interpolymer interaction was developed. The effects of functional groups content and acrylate unit on interpolymer interaction were studied with this parameter combined with light scattering. The results show that interpolymer interaction ability increases with the functional group and long chain alkyl acrylate content. With the increase in side chain length of acrylate unit, enhancement in the interpolymer specific interaction can be realized.