The preparation of olefins from steam cracker or refinery C.sub.4 streams is carried out by selective hydrogenation of butadienes and acetylenic impurities in the steam cracker or refinery C.sub.4 stream, with simultaneous or subsequent, at least partial isomerization of 1-butene to 2-butene, followed by removal of i-butene from the C.sub.4 stream by reaction with an alcohol to form an ether, followed by removal of oxygen-containing impurities from the C.sub.4 stream using adsorber materials, followed by two-stage metathesis of the butenes in the C.sub.4 stream by conversion of 1-butene and 2-butene present in the C.sub.4 stream into propene and 2-pentene and subsequent reaction of the 2-pentene with ethene in the presence of a metathesis catalyst to form propene and 1-butene. Optionally, butadiene may be removed from the C.sub.4 stream by extractive distillation in a preliminary step.