Fragments of a Portland cement clinker were examined by scanning electron microscopy with backscattered electron imaging and X-ray microanalysis. In some, the microstructure was normal, but in others varying proportions of material having features characteristic of formation in a reducing atmosphere were present. The principal such features were partial decomposition of the alite and the presence in the interstitial matrix of a high proportion of aluminate and small amounts of sulfide and metallic iron. The alite decomposition, which had produced myrmekitic intergrowths of belite and lime, appeared to have been preceded by incorporation of S2- and additional Fe2+ into the structure, which had lowered its stability, and to have occurred under reducing conditions. The mechanisms of the alite decomposition and of the formation of the interstitial material are discussed. All the fragments showing features characteristic of formation under reducing conditions were black; a dark colour in clinker is thus not a safe indication that such features are absent.