The adsorption of a homologous series of mono- and dibasic aliphatic acids by acid and alkali-condensed phenolic resins and by an amino-resin has been studied. In acid-condensed phenolic resins, the adsorption in a homologous series increases with in­crease in molecular weight, while in alkali-catalysed phenolic resins and amino-resins the order of adsorption is reversed. The adsorption of substituted acetic acids by an amino-resin has also been determined and the influence of the various substituents on adsorption studied. The introduction of acidic groups increases adsorption, while that of basic groups decreases it. The effect of the amino group is much more pronounced than that of hydroxyl group.