A process for the preparation of methyl tertiary butyl ether (MTBE) in two stages:the first stage comprising the reaction of isobutene with methanol in the presence of an acid catalyst, yielding MTBE, while the second stage comprises firstly the conversion of normal-butenes present in the hydrocarbon flow, which remains behind after separation of the MTBE formed in the first stage into isobutene and secondly passing the mixture thus obtained to a reaction zone to form MTBE.Optionally an extra amount of methanol can be supplied to said reaction zone. Said reaction zone can be the same as that used in the same first stage or another one.