AbstractThe various factors affecting the accuracy and procedure of hydroxyl determination are discussed. Phenolic acetates have been found to hydrolyse fairly rapidly during the hydrolysis of anhydrides, a process which is normally carried out at the end of the acetylation reaction, and this is held to account for anomalous hydroxyl values obtained previously for phenolic substances.A satisfactory semi‐micro procedure has been developed for use with samples of phenols (20 to 80 mg.).In determining the acid values of phenols and phenolic‐resin intermediates the use of phenolphthalein as indicator is to be avoided. Neutral red is suggested as a suitable indica