Abstract: The present invention relates to a process for the preparation of diols of the general formula (I) ##STR1## and monoesters thereof of the general formula (II) ##STR2## and the general formula (III) ##STR3## wherein R.sub.1 and R.sub.2 are the same of different groups and each denotes a lower alkyl group having 1 to 4 carbon atoms, preferably a methyl group. The process of this invention comprises condensing in a multi-phase reaction environment an aldehyde containing a hydrogen atom in the alpha position and having the general formula (IV) ##STR4## wherein R.sub.1 and R.sub.2 having the meaning given above, in the presence of a catalytic system consisting of a phase transferring catalyst and an alkaline substance.