The present invention relates to a method for the preparation of morphine compounds comprising a low content of α,β-unsaturated compounds, which comprises the steps of: (i) bringing the crude morphine compound into contact with a base, at a pH of greater than 13, under conditions which make possible the Michael addition reaction on the α,β-unsaturated compound(s) present; (ii) separating the morphine compound from the reaction mixture; and (iii) if appropriate, separating the addition product formed from the morphine compound. It also relates to a composition comprising at least 99% by dry weight of morphine compound or of a pharmaceutically acceptable salt thereof, and an α,β-unsaturated compound in a content of less than 100 ppm.