Formaldehyde present in a waste water containing lower carboxylic acids is recovered by reacting said waste water at 0.degree. to 90.degree. C. with the amount of ammonia required for converting said formaldehyde into hexamine and the latter is recovered in solid form by evaporating the resulting solution.The solid hexamine thus recovered has a content in lower carboxylic acids not exceeding 0.05 wt.%, is practically free from other impurities and is suited for any conventional use.