A fibre-optic sensor for the monitoring of hazardous exhaust gases is described. The sensor based on Differential Optical Absorption Spectroscopy was developed to operate within exhaust environments, monitor several exhaust gases and demonstrate low susceptibility to interferences from other gases. Experimental results describing the calibration of the sensor against a commercial analyser and tests documenting the sensor's operating capabilities within the exhaust of an engine are presented. The lower limit of detection for the sensor was found to be 5ppm for nitric oxide, and 1ppm for both nitrogen dioxide and sulphur dioxide. Response times were found to be 3.4 seconds. © 2007 IOP Publishing Ltd.