The invention relates to an X-ray source, comprising a housing, an X-ray tube which is arranged therein and which is surrounded by a liquid insulating medium, and a temperature sensor for determining the temperature within the liquid insulating medium. A simple embodiment of the temperature sensor consists in that it comprises a measuring element which is in thermal contact with a spatial zone in the insulating medium and which assumes a uniform temperature which is dependent on the temperature distribution in the zone during operation of the X-ray source, and a temperature transducer which is thermally coupled to the measuring element and which generates an electric signal which is dependent on the temperature of the measuring element.