The estimation of the compatibility of different pairs of polymers can, be based on capillary viscometry data for ternary polymer-polymer-solvent systems using mathematical models based on the slope of the Huggins equation (Delta b) and Huggins constant (Delta k＇), In this study, the compatibility of binary mixtures of six types of methacrylic copolymers with similar molecular weights but different functional groups [one characterized by amine groups (EuE), two by ammonium groups (EuRL EuRS), two by carboxylic groups (EuL EuS), and one without charge (EuNE)] was evaluated using these methods. On the basis of Huggins and Kraemer constants, acetone and tetrahydrofurane were selected as good solvents for the programmed blends. Cationic copolymers mixed with anionic copolymers showed the formation of visible aggregates. The study performed on the other blends showed that EuRL and EuRS could be considered compatible with EuE; EuNE was incompatible with both EuL and EuE, EuL and EuS were incompatible between them. EuRL and EuRS could be considered compatible even if the weight ratio seems to influence the behavior of the two copolymers. The Delta k＇ approach seems to be more robust than the Delta b model. The compatibility of different pairs of methacrylic copolymers with similar molecular weights could be evaluated using capillary viscometry, (C) 2000 John Wiley & Sons, Inc.