PURPOSE: To obtain a polyhydric alcohol without removing formaldehyde by distillation in high yield, by heating a high b.p. compound, formed as a by-product in the preparation of 4,4-dimethyl-1,3-dioxane, with a tertiary butanol.

CONSTITUTION: A compound (Residol) having a higher b.p. than dioxane, which is formed as a by-product in the preparation of 4,4-dimethyl-1,3-dioxane from isobutene and formaldehyde, is heated at 120-180°C with a tertiary butanol and/or isobutene in the presence of boric acid or a boron compound containing oxygen to give 3-methylbutane-1,3-diol together with triols of formulas I and II. The reaction scheme is shown for a component of the Residol. Water added is less than the equivalent amount of the water needed for the hydrolysis of the Residol. USE;E.g. strarting meterial for the preparation or an additive of polymers, plasticizer, wetting agent, etc.