A method and device for determining insulation resistances in a motor vehicle. The device includes a control device which, in at least two measuring intervals, controls a respective operating state of at least one power converter of the motor vehicle, which is conductively connected to the traction energy store. The device further comprises a measuring device having a measuring terminal, which is conductively connected or connectable to least one DC voltage pole of an electric traction energy store of the motor vehicle, and a ground terminal which is conductively connected or connectable to a reference potential of the motor vehicle. The measuring device, in the at least two measuring intervals, respectively measures a conductance between the measuring terminal and the ground terminal. The device further comprises a calculation device, which determines the insulation resistances as a function of the at least two measured conductances and the operating states controlled.