The binding of Ce(III), Eu(III), and Am(III) to a weak carboxylic acid ion exchange resin was measured at several temperatures. The thermodynamic values correspond to contact ion-pair formation of the trivalent cations with the carboxyllc sites. The results of this study of binding of lanthanides and actinides to a weak carboxyllc acid resin are compared to analogous data for a strong sulfonic acid ion exchange resin.