In a method for controlling a vehicle with a drive system comprising a power unit configuration adapted to provide power for the vehicle's operation, and further comprising a planetary gear and a first and second electrical machine, connected to components in the planetary gear via their rotors, a locking means is moved from a release position, in which the planetary gear's components are free to rotate independently of each other, to a locked position, in which two of the planetary gear's components are locked together, so that the three components in the planetary gear rotate with the same speed. The power unit configuration is controlled in order to achieve a synchronous, or substantially synchronous, rotational speed between the input and output shaft of the planetary gear, and the locking means are then moved to the locked position.