A method and a device for monitoring the insulation resistance in an ungrounded electrical network having a constant-voltage d.c. link and at least one inverter, connected to it, for controlling an n-phase electrical consumer in an n-phase network. A voltage to be monitored, is determined during operation of the consumer, which represents a voltage fluctuation of supply voltage potentials of the constant-voltage d.c. link with respect to a reference potential. In addition, a variable characterizing an electrical frequency of the electrical consumer is determined, particularly an electrical angular speed of the electrical consumer. A first spectral amplitude of the voltage to be monitored at the n-fold electrical frequency of the electrical consumer, is compared to a first reference value, and detects a symmetrical insulation error in the constant-voltage d.c. link or the n-phased network, if the comparison yields a deviation of the first spectral amplitude from the first reference value.