Frontal analysis continuous capillary electrophoresis was used to measure the binding of beta -lactoglobulin (BLG) to sodium poly( styrenesulfonate)( PSS) and sodium poly( 2-acrylamido-2-methylpropanesulfonate) (PAMPS), two strong polyanions with similar linear charge densities. The binding isotherms obtained were well-fit by the McGhee -von Hippel equation, yielding the intrinsic binding constant, K-obs, and the binding site size, n, representing the number of polymer segments per bound protein. Two opposite ionic strength (I) dependencies of K-obs for BLG-PSS were found depending upon pH, that is, increase of K-obs with I at pH 7.0, and decrease of K-obs, with I at pH 6.3. The opposite I dependencies reflected the roles of electrostatic interactions for systems with heterogeneously charged components, but also demonstrated the inapplicability of a simple formulation (log K-obs = log K-o - Z(phi) log [M+]) put forward for the binding of protein to DNA. K-obs for PAMPS was always much smaller than that for PSS at equal pH. In addition, n for BLG-PSS was small and independent of I and pH, while n for PAMPS was large and increased with I and pH, both results consistent with "tighter" binding of BLG to PSS than to PAMPS. This marked contrast may arise from the effects of polymer persistence length or from hydrophobic interactions.