The paper investigates the hydrodynamic properties of polyacrylonitrile (PAN) and poly(N-(4-carboxyphenyl)malcimide) (PMI) solutions in dimethylformamide (DMF) in comparison with PMI/PAN/DMF ternary mixtures. The experimental data obtained by viscometry have been discussed by means of two methods: first, the plots obtained with the classical Huggins equation are analyzed, and in parallel, an evaluation of the parameters given by the new Wolf model is presented. The experimental data obtained for binary polymer/ solvent and ternary polymer/polymer/solvent mixtures fit well with this last method and allow the calculation of intrinsic viscosities and other hydrodynamic parameters, which provide new information about the competition between different types of interactions for polymer mixtures in solution. The compatibility of the two polymers dissolved in a common solvent is also discussed on the basis of two parameters: Delta b and alpha. reflecting the interactions between the polymer segments and polymer/polymer miscibility. respectively. The sign of these parameters shows that the PMI/PAN blends in DMF are miscible in the range of PAN mass fraction between 0.36 and 0.75.