The phase behaviour of Fe and Co particles, supported on CaCO3 was studied using mainly in situ PXRD and Mossbauer spectroscopy. This is of interest as these systems can be used as catalysts to produce multi-walled carbon nanotubes (MWCNTs) following appropriate heat treatment and reduction. The Fe-Co/CaCO3 samples were prepared by deposition of the metallic particles from solution onto the support. The study indicated that the CaCO3 support reacted in air or N-2 with the iron and cobalt present in all three samples studied (10% Fe/CaCO3, 10% Co/CaCO3 and 10% Fe-Co/CaCO3) to give a brownmillerite structure with the general formula Ca2M2O5 (M = Fe, Co or Fe + Co). Reduction of the bimetallic catalyst gave Fe2Co alloy and Co. This confirms earlier reports by other authors that implicated this alloy as the active phase in the synthesis of the MWCNTs. The bimetallic catalyst was easier to reduce than the monometallic catalysts. (C) 2014 Elsevier Ltd. All rights reserved.