Poly(isobutene-alt-maleic acid)s modified with p-tert-butylphenyl or adamantyl groups interact with beta-cyclodextrin in water due to inclusion of the hydrophobic substituents in the beta-cyclodextrin cavity. The formation of these host-guest complexes was investigated in dilute aqueous solution using affinity capillary electrophoresis. The electrophoretic mobility of the polymers was measured in the presence of increasing concentration of beta-cyclodextrin in the capillary. The binding constants K (expressed per mole of beta-cyclodextrin) were determined from the change of the electrophoretic mobility of the polymers with the increase of P-cyclodextrin concentration. K = (2.8-3.9) x 10(4) M-1 for p-tert-butylphenyl guest polymers and K = (2.2-3.5) x 10(3) M-1 for adamantyl guest polymers. These findings are consistent with titration microcalorimetry data, demonstrating that capillary electrophoresis is a suitable technique to quantitatively measure interactions with hydrophobically modified polymers. The change in electrophoretic mobility upon complexation can be explained from a transition of the free polymers, which form compact unimer globules, into bound polymers forming extended random coils.