A novel phosphor coated glassware based technique for measuring liquid temperature up to 120°C has been investigated and developed. The active coating consists of a mixture of two phosphor powders which is applied to three different test vessels. One test vessel containing 40 ml of water is heated by a heating coil and its phosphor coating is connected by two optical fibres positioned directly beneath the external base of the beaker, which are used to transmit and receive light. The remaining two test vessels are individually placed in an industrial free electron laser (IFEL), which provides the necessary microwave radiation for heating liquids. Thermal quenching dominates the luminescence characteristics of one phosphor while the second phosphor does not undergo thermal quenching. Ratios of their emission intensities are calculated and analysed. The resulting ratio decreases linearly as temperature increases. The results presented indicate a temporal response of 1 s as well as good repeatability when calibrated against a thermocouple in the laboratory. This paper also describes testing of the coating and its application in monitoring liquid temperature in an IFEL. © 2005 IOP Publishing Ltd.