An internal combustion engine (1) has a fuel tank (18) and a fuel vapor reservoir (25). According to the method during the tank vent period—before opening the tank vent valve (28) a reference value is updated if a condition is met, and otherwise the existing reference value is maintained, —before opening the tank vent valve (28) an opening period is determined as a function of at least one variable, —the tank vent valve (28) is controlled such that the flow achieves the target flow value by the end of the opening period that was determined, —the load level of the fuel vapor reservoir (25) is computed based on the reference value, —a fuel amount is computed based on the load level, —and the fuel amount to be metered by the injection valve (22) is corrected based on the computer fuel amount.