In a procedure for the functional diagnosis of an activateable fuel tank ventilation valve of the fuel tank system of an internal combustion engine, especially of a motor vehicle, whereby in specifiable time intervals when the fuel tank ventilation valve is activated to open, regeneration gas is added to the air drawn into the combustion chamber, whereby fuel is delivered to the combustion chamber, whereby the fuel, the air, respectively the fuel, the air and the regeneration gas are combusted in the combustion chamber and whereby by comparison of at least one operating parameter, which characterizes the combustion of fuel and intake air with the corresponding operating parameter, which characterizes the combustion of fuel, intake air and regeneration gas, inference can be made about the functional capability of the fuel tank ventilation valve, the HC-concentration of the regeneration gas is specifically affected by changing the regeneration gas stream.