A process for preparing tri- and tetraoxymethylene glycol dimethyl ether (POMDME.sub.n=3,4) by reacting formaldehyde with methanol and subsequently working up the reaction mixture by distillation, consisting of: a) feeding aqueous formaldehyde solution and methanol into a reactor and reacting to give a mixture including formaldehyde, water, methylene glycol (MG), polyoxymethylene glycols (MG.sub.n>1), methanol, hemiformals (HF), methylal (POMDME.sub.n=1) and polyoxymethylene glycol dimethyl ethers (POMDME.sub.n>1); b) feeding the reaction mixture a into a first distillation column and separating into a low boiler fraction b1 and a high boiler fraction b2 including formaldehyde, water, methanol, MG.sub.n>1, HF and POMDME.sub.n>1; c) feeding the high boiler fraction b2 into a second distillation column and separating into a low boiler fraction c1 including formaldehyde, water, MG, MG.sub.n>1, methanol, HF, di-, tri- and tetraoxymethylene glycol dimethyl ether (POMDME.sub.n=2,3,4), and a high boiler fraction c2; d) feeding the low boiler fraction c1 into a third distillation column and separating into a low boiler fraction d1 and a high boiler fraction d2 substantially consisting of formaldehyde, water, MG, MG.sub.n>1 and POMDME.sub.n=3,4; e) feeding the high boiler fraction d2 into a phase separation apparatus and separating into an aqueous phase e1 substantially consisting of formaldehyde, water, MG and MG.sub.n>1, and an organic phase e2 including POMDME.sub.n=3,4; and f) feeding the organic phase e2 into a fourth distillation column and separating into a low boiler fraction f1 substantially consisting of formaldehyde, water, MG and MG.sub.n>1, and a high boiler fraction f2 substantially consisting of POMDME.sub.n=3,4.