Five perovskite oxides with the composition La1-x KxMnyFe1-yO3 (0 less than or equal to x, y less than or equal to 0.2) were synthesized by the co-precipitation method. Techniques such as chemical and XR fluorescence analysis, powder X-ray diffraction (XRD), BET surface area, IR, EPR and Mossbauer spectroscopy were applied to characterize the structural features of the perovskites. The activity of the solids was strongly dependent on the presence of iron carbides. It was observed that K promotes the reducibility of the solid to alpha-Fe giving rise to more carbide phases. Mn produces a significant increase in alkenes production. The combined presence of both K and Mn (La0.9K0.1Fe0.9Mn0.1O2.9) strongly modified the selectivity of the oxides, giving rise to a stable catalyst with high yield of C-2-C-4 alkenes. (C) 2002 Elsevier Science B.V. All rights reserved.