Process for continuous preparation of homo- and copolyoxymethylenes with stable end groups in a homogeneous phase, in which in a tubular reactor with static mixing elements cyclic formaldehyde oligomers are polymerized in the presence of protonic acids as initiators in amounts of 0.005 to 500 ppm in a polymerization zone, in which the residence time of the reaction components in this zone is 0.1 to 10 min and in which the initiator is deactivated immediately after polymerization, there being a fluid transition between the polymerization and deactivator zones which is determined solely by the addition of deactivator, and in which, besides deactivation, stabilization of the chain ends is carried out in the presence of residual monomers, with the volatile constituents being removed from the reaction mixture in a subsequent vent unit. Stabilization is carried out in the presence of alkaline substances, either at temperatures of 150.degree. C. to 250.degree. C., if necessary in the presence of water or primary alcohols, or at 175.degree. C. to 250.degree. C. in the absence of these additives.