Abstract: A method of and a device for measuring the temperature of the axle bearing of a vehicle, especially of a rail vehicle while moving. The device is mounted adjacent a rail on which the vehicle runs and receives radiation from beneath the axle bearing over a strip-like range thereof of predetermined size. A detector receives the radiation after the radiation has been angularly diverted and develops an electrical signal in conformity with the value of the radiation. A modulator or interrupter ahead of the detector provides for an alternating, or pulsating, signal which is then amplified in an electronic circuit. For comparison purposes, a source of infra red radiation of known value is provided whereby the calibration of the device can be checked.