The sensor system comprises an implantable sensor and a user device associated with it. A sensitive liquid is enclosed in the sensor, into which glucose can penetrate. The viscosity of the mixture of sensitive liquid and glucose is measured. The user device, which controls the measurement and its evaluation, is a portable device worn externally on the skin. The viscosity is measured based on the rotation of a measuring element driven by a driving magnet, both disposed in the sensor. The rotation of the measuring element is analyzed based on its decay behavior following switch-off of the driving magnet. In a second embodiment, the viscosity is measured based on the oscillatory behavior of an oscillating element excited to oscillate by a magnet, both disposed in the sensor. The oscillatory behavior is analyzed based on the decay behavior of the oscillating element following switch-off of the magnet.