Compound catalysts involving methanol-synthesis/methanol-conversion functions offer promise for selective conversion of synthesis gas into C-2-C-3 hydrocarbons and particularly of C-2-C-3 olefins. This approach was followed, by modifying a ZSM5 zeolite with the addition of P compounds. The catalyst was characterized using SEM-EDX, BET, XRD and TPD. To elucidate the reaction mechanism, experiments were conducted in a Berty well mixed reactor to establish the following : a) methanol is a key intermediate present in minute amounts, b) water gas shift reaction takes places under conditions of equilibrium, c) hydrogenation and condensation of light olefins into higher hydrocarbons is mild and allows for significant fractions of C-2-C-3 olefins to be produced.