Lava samples, collected periodically during the 1971 eruption of Mt Etna, have been analysed. A certain evolution of their composition has been observed: the first lavas are phonolitic tephrites, while the last ones are mugearites. This evolution can be explained by assuming a pneumatolytic differentiation in the uppermost parts of the magma column and a subtraction of femic phenocrysts by gravitational differentiation in its deeper parts, where the last products originated. Furthermore, the analyses of the 1971 lavas are compared with all available data of ancient products of this complex volcano and, particularly, with those of its historical eruptions.