The invention concerns a process and an apparatus for preparing tertiary ethers. According to the process, C.sub.4 to C.sub.6 isoolefins and possibly heavier olefins contained in the feedstock are reacted with lower aliphatic alcohols, in particular methanol or ethanol, in a catalytic distillation reactor system in order to produce the corresponding ethers. According to the invention, the reaction between the isoolefins and the alcohols is essentially carried out in at least one reactor (5-7) of the kind, which is combined with a distillation column (3) intended for product separation, by conducting at least a part of the liquid flow of the column through the reactor and returning it to a lower tray than the one from which it was taken. At least half of cation exchange resin is placed in the side reactor (5-7). In the process according to the invention, the catalyst can be rapidly changed without stopping the process. In order to improve conversion additional alcohol can be fed to at least one of the side reactors.