Abstract: Apparatus for detecting alcohol vapour in a gas, such as exhaled breath. The apparatus comprises a detachable mouthpiece through which a person exhales. The mouthpiece includes a vent for venting some exhaled breath to atmosphere and an outlet port connected to a sensor which provides an electrical output dependent upon the amount of alcohol in the breath sample. A piston and cylinder valve assembly is provided for introducing a predetermined amount of the sample into the sensor at a time dictated by an operator. A calibrated meter, which may be analogue or digital is connected to the sensor to indicate the amount of alcohol in the sample.