This paper presents monitoring of ageing in high power insulated gate bipolar transistor (IGBT) modules subjected to sinusoidal loading at nominal power level. On-state voltage for IGBT, diode, and rise in interconnection resistance are used as ageing parameters. These are measured in three different ways: calibration of power modules after 24 h of operation, offline characterization every 5 min of operation, and continuous measurement during normal converter operation. Four power modules are tested, which are cycled to different degradation levels by number of cycles, where one is tested until failure. The characterization at different stages of lifetime indicates that the rise in resistance originates from thermo-mechanical degradation of interconnects. Post-test investigations: four-point probing and micro-sectioning indicate thermo-mechanical induced degradation of the chip topside interconnects as the source for rise in resistance. Major degradations are observed in bond wires and metallization on the low side diode.