Method for adapting a combustion engine of a hybrid drive system of a motor vehicle to a gaseous fuel of an unknown fuel quality, wherein the hybrid drive system comprises an electric machine that can be mechanically coupled to the combustion engine , comprising the following steps: - driving the combustion engine at a drag speed by means of the electric machine of the hybrid drive system , wherein during driving of the combustion engine , a drag torque of the combustion engine is determined by means of the electric machine , wherein the determined drag torque is taken into account when determining the fuel quality of the gaseous fuel, - introducing the gaseous fuel into the combustion engine by means of a fuel inlet device of the hybrid drive system , - igniting the gaseous fuel by means of an ignition device of the hybrid drive system , - determining an ACTUAL operating torque of the combustion engine by means of the electric machine mechanically coupled to the combustion engine , - comparing the determined ACTUAL operating torque with a TARGET operating torque of the combustion engine that is to be expected given use of a fuel with a known fuel quality, by means of a comparison device , - defining a corrective value for adapting the ACTUAL operating torque of the combustion engine to the TARGET operating torque by means of an engine control device of the hybrid drive system , and - adapting the ACTUAL operating torque of the combustion engine to the TARGET operating torque on the basis of the corrective value by means of the motor control device of the hybrid drive system , wherein during the determination of the ACTUAL operating torque, the combustion engine is held at the drag speed by means of the electric machine . - driving the combustion engine at a drag speed by means of the electric machine of the hybrid drive system , wherein during driving of the combustion engine , a drag torque of the combustion engine is determined by means of the electric machine , wherein the determined drag torque is taken into account when determining the fuel quality of the gaseous fuel, - introducing the gaseous fuel into the combustion engine by means of a fuel inlet device of the hybrid drive system , - igniting the gaseous fuel by means of an ignition device of the hybrid drive system , - determining an ACTUAL operating torque of the combustion engine by means of the electric machine mechanically coupled to the combustion engine , - comparing the determined ACTUAL operating torque with a TARGET operating torque of the combustion engine that is to be expected given use of a fuel with a known fuel quality, by means of a comparison device , - defining a corrective value for adapting the ACTUAL operating torque of the combustion engine to the TARGET operating torque by means of an engine control device of the hybrid drive system , and - adapting the ACTUAL operating torque of the combustion engine to the TARGET operating torque on the basis of the corrective value by means of the motor control device of the hybrid drive system , wherein during the determination of the ACTUAL operating torque, the combustion engine is held at the drag speed by means of the electric machine . <EOS>