In these processes, the TBA content increases and the isobutene content decreases from reactor to reactor as a consequence of the progress of the reaction. The composition of the reaction mixture approaches, at a decreasing rate, the thermodynamic equilibrium between water, isobutene and TBA, so that complete conversion cannot be achieved. Starting from technical-grade isobutene streams, for example raffinate I, conversions of only approx. 82% are achieved