Thermal oil system containing thermal oil, which forms a momentary reference volume (V r) at a first temperature (T 1) and a first point in time and forms an actual momentary operating volume (V a) during operation of the thermal oil system at a second temperature (T 2) and a second point in time that is temporally offset from the first point in time by a differential time period, characterised • in that a volumetric change measuring device is provided which comprises a volumetric flow rate measuring device that determines an actual volumetric change (V a) occurring over the differential time period between the momentary reference volume (V r) and the actual momentary operating volume (V a) of the thermal oil by the volumetric flow rate measuring device determining the volumetric flow rate of the thermal oil occurring over the differential time period in a line of the thermal oil system, • in that a temperature measuring device is provided which determines a temperature difference (ΔT) between the first temperature (T 1) and the second temperature (T 2) of the thermal oil, • in that a target operating volume calculation device connected to the temperature measuring device is provided which, on the basis of the momentary reference volume (V r) and the determined temperature difference (ΔT) of the thermal oil, determines a momentary target operating volume (V n) of the thermal oil, associated with the second point in time, • in that a target operating volume comparison device connected to the target operating volume calculation device is provided which determins a target volumetric change (V n) of the thermal oil for the differential time period by comparing the momentary reference volume (V r) of the thermal oil and the momentary target operating volume (V n) of the thermal oil, • in that a volumetric change comparison device is provided which is connected to the volumetric change measuring device comprising a volumetric flow rate measuring device and to the target operating volume comparison device, and compares the actual volumetric change (V a) of the thermal oil and the target volumetric change (V n) of the thermal oil with one another, and • in that an evaluation device connected to the volumetric change comparison device is provided which detects a leak in the thermal oil system if the actual volumetric change (V a) of the thermal oil differs from the target volumetric change (V̇ n) of the thermal oil by a volumetric change difference (Δ V̇) that indicates a leak in the thermal oil system. • in that a volumetric change measuring device is provided which comprises a volumetric flow rate measuring device that determines an actual volumetric change (V a) occurring over the differential time period between the momentary reference volume (V r) and the actual momentary operating volume (V a) of the thermal oil by the volumetric flow rate measuring device determining the volumetric flow rate of the thermal oil occurring over the differential time period in a line of the thermal oil system, • in that a temperature measuring device is provided which determines a temperature difference (ΔT) between the first temperature (T 1) and the second temperature (T 2) of the thermal oil, • in that a target operating volume calculation device connected to the temperature measuring device is provided which, on the basis of the momentary reference volume (V r) and the determined temperature difference (ΔT) of the thermal oil, determines a momentary target operating volume (V n) of the thermal oil, associated with the second point in time, • in that a target operating volume comparison device connected to the target operating volume calculation device is provided which determins a target volumetric change (V n) of the thermal oil for the differential time period by comparing the momentary reference volume (V r) of the thermal oil and the momentary target operating volume (V n) of the thermal oil, • in that a volumetric change comparison device is provided which is connected to the volumetric change measuring device comprising a volumetric flow rate measuring device and to the target operating volume comparison device, and compares the actual volumetric change (V a) of the thermal oil and the target volumetric change (V n) of the thermal oil with one another, and • in that an evaluation device connected to the volumetric change comparison device is provided which detects a leak in the thermal oil system if the actual volumetric change (V a) of the thermal oil differs from the target volumetric change (V̇ n) of the thermal oil by a volumetric change difference (Δ V̇) that indicates a leak in the thermal oil system.