In-situ Mossbauer spectroscopy was used to study FexCo1-xMoO4 molybdates and their mixtures with Bi-2(MoO4)(3) in the 360-415 degrees C range under a flow of a mixture C3H6/O-2/N-2 = 100/100/560 Torr. Ferrous molybdate FeMoO4 was progressively oxidized to ferric molybdate and ferric oxide. Mixed cobalt and iron molybdates Fe-x/Co1-xMoO4 exhibited the following behaviour : (1) for all iron contents, the CY phase spectrum diminished to the benefit of the beta phase. The Fe3+ sites of the solid solution underwent reduction. At high iron content, the Fe3+ in Fe-2(MoO4)(3) was detected and diminished by half under catalytic conditions; (2) when Bi-2(MoO4)(3) was added to FexCo1-xMoO4, there was no reduction of Fe3+ sites in the solid solution. The alpha --> beta phase transition was not observed in the case of the low and medium iron contents. In the high iron content samples the alpha-phase spectrum decreased to the expense of the beta phase but did not disappear totally after catalytic reaction. It is concluded that cobalt stabilizes Fe2+ sites in the solid solution while Bi-2(MoO4)(3) stabilizes Fe3+ in the solid solution, It is proposed that Fe2+-Fe3+ t pairs take an important place in the mechanism of propene mild oxidation.