Résumé : CO hydrogenation over CoCu based catalysts containing added amounts of either Mn or Mg, was studied by chemical transient response. Both, the surface build-up by abrupt CO partial pressure changes and the emptying of the surface by desorption were followed. Calculated amounts of surface oxygen and carbon suggest a formate species to be an important reaction intermediate. A quantitative analysis demonstrates the Mn containing catalyst to be more active and selective to longer-chain hydrocarbon formation. Despite a similar desorption behaviour of the active intermediates on both types of catalysts, the relaxation experiments indicate the presence of a more stable (spectator) complex, [CO2*], on MnCoCu. This complex regenerates the active “ masi ” with a large time constant thus controlling the time constants of all the products formed thereafter (CH4, H2O, CO2, CO). In the proposed mechanism, the active surface under working conditions is definitely not in a metallic state.