The present invention relates to a solid, acid catalyst for the preparation of polytetrahydrofuran, polytetrahydrofuran copolymers, diesters or monoesters of these polymers by polymerization of tetrahydrofuran in the presence of at least one telogen and/or comonomer, which has a BET surface area of at least 160 m.sup.2/g and an acid center density of at least 0.05 mmol/g for pK.sub.a values of from 1 to 6, to a process for preparing it and to a process for the polymerization of cyclic ethers over this catalyst.