We propose an experiment on POLARIS to elucidate the changing structure and stoichiometry of mixed metal quaternary molybdenum nitrides, Con(1-x)FenxMonN (n=3, 6; 0 ≤ x ≤ 1). Testing of catalytic activity (in the synthesis of ammonia) has shown that activity declines markedly from Co3Mo3N to Fe3Mo3N, tied to the observation that the reversible denitrogenation¿nitrogenation in the Co-Mo-N system (Co3Mo3N ↔ Co6Mo6N) finds no parallel for Fe-Mo-N. We therefore propose to investigate (ex-situ) mixed (Co,Fe)3Mo3N nitrides through the (de)nitrogenation cycle to correlate structure, stoichiometry and activity and search for the origin of this striking contrast in chemical behaviour.