The invention relates to a process for the preparation of ethylene and/or propylene comprising: (a) an oxygenate conversion step wherein a gaseous effluent comprising olefins is obtained; (b) subjecting the effluent to water removal and compression steps; (c) acid gas removal from the effluent obtained in step (b), wherein the gaseous effluent is treated with a caustic solution and a non-aqueous liquid stream comprising aromatic C7+ hydrocarbons is added to the caustic solution to control the formation of red oil, to obtain a treated gaseous effluent and a spent liquid phase comprising spent caustic solution, aromatic C7+ hydrocarbons and high molecular weight contaminants; and (d) separating an aqueous phase and a non-aqueous phase from the spent liquid phase; (e) purifying the non-aqueous phase by membrane separation using a hydrophobic non-porous or nano-porous membrane; and (f) separating the olefinic product from the treated gaseous effluent.