The invention concerns a method for optimizing the operation of a reaction section for hydrocarbon synthesis starting from a feed comprising synthesis gas, operated in the presence of a catalyst comprising cobalt. This method comprises the following steps: a) determining the theoretical molar ratio, PH2O:PH2, in the reaction section; b) optionally, adjusting the ratio PH2O:PH2 determined in step a) to a value strictly below 1; c) determining the new value for the theoretical ratio PH2O:PH2 in the reaction section; and repeating steps a) to c) until the ratio of the partial pressures of water and hydrogen, PH2O:PH2, has a value strictly less than 1.1.