Process for preparation of polyoxymethylenes via polymerization of the monomers a) in the presence of cationic initiators b), and also, if appropriate, in the presence of regulators c), and subsequent deactivation and isolation of the polymer, which comprises undertaking the polymerization in a tubular reactor with static mixing elements and with a mixing zone, a polymerization zone, and a deactivation zone, the diameter of the tubular reactor in the mixing zone being