The reaction of 2-methyl-1,8-octanedial with formaldehyde is carried out under substantially the same conditions as used for the reaction of 1,9-nonanedial with formaldehyde in the above-mentioned process B. The product separation from the thus-obtained hydroxymethylation reaction mixture is effected, for example, by extracting said reaction mixture with an organic solvent, such as diethyl ether.