A series of iron-lithium mixed molybdates of the general formula LixFe1-xMoO4 (x = 0.45-0.55) was synthesized and characterized by XRD and Mossbauer spectroscopy. The modification of this system by a Li to Fe ratio variation at constant (Li + Fe) content gives an opportunity to study structure-activity relations in catalytic methane oxidation to formaldehyde. Other products registered by means of gas chromatography are ethane and carbon oxides. Reduction and oxidation of the samples was also studied. The influence of stoichiometry of the mixed molybdates on their structure, ion state in the lattice and consequently on their catalytic performance was evaluated.