The free hydroxyl groups may be present as polyhydric alcohols, partial esters, polyester end groups or hydroxylated fatty acids. The method of test is not applicable to resin containing both hydroxyl groups and epoxide groups, because the latter will also be attacked and included in the result. The principle consists in acetylation of the groups with anhydride, hydrolysis of the exess acetic anhydride and titration of the resulting acetic acid with potassium hydroxide solution, either in the presence of a colour indicator or potentiometrically. The method is not applicable to cellulose nitrate or phenolic resins.