The invention is a method for processing a mixture containing water, 3-methyl-1-butene and at least one other methylbutene. The method comprises primary distillation of the mixture, giving a gaseous primary overhead product containing methylbutene and water and a water-free primary bottom product containing 3-methyl-1-butene; condensation of the gaseous primary overhead product so as to give a condensate comprising a liquid aqueous phase and a liquid organic phase; separation of the condensate into a liquid aqueous phase and a liquid organic phase; discharge of the liquid aqueous phase; recirculation of the organic phase to the primary distillation; and finally secondary distillation of the water-free primary bottom product from the primary distillation so as to give a secondary overhead product comprising 3-methyl-1-butene and a secondary bottom product. The secondary overhead product obtained has a purity which enables it to be used directly as monomer or comonomer for preparing polymers or copolymers.