A process for preparing polyisobutenyl (thio)ethers by reacting a polyisobutene epoxide having at least one terminal epoxide group with itself, with other epoxides and/or with nucleophiles selected from among alcohols and thiols in the presence of a Lewis-acid compound as catalyst and/or a cationic photoinitiator with illumination is described. The side reaction of the rearrangement of the epoxide to the polyisobutenyl aldehyde is suppressed. In addition, a curable composition comprising a polyisobutene epoxide, an epoxide different therefrom and optionally a poly(thi)ol is also described.