The present invention provides a process for changing the given mean molecular weight Mn in the continuous preparation of polytetrahydrofuran or tetrahydrofuran copolymers, the mono- or diesters of polytetrahydrofuran or of tetrahydrofuran copolymers by polymerizing tetrahydrofuran in the presence of a telogen and/or of a comonomer over an acidic catalyst, wherein a) the molar ratio of telogen to tetrahydrofuran or to tetrahydrofuran and comonomer is changed, b) then the mean molecular weight of at least one sample is determined, c) until the mean molecular weight thus determined differs from the molecular weight to be achieved by the change, the already formed polytetrahydrofuran or the tetrahydrofuran copolymers, the mono- or diesters of polytetrahydrofuran or tetrahydrofuran copolymer is at least partly depolymerized over an acidic catalyst and d) the tetrahydrofuran recovered by depolymerization is recycled at least partly into the polymerization.