A breathalyser with a pressure sensor (2) for recording a signal representative of the breathing air flow, an electrochemical sensor (1) for recording a signal representative of the alcohol content of the breathing air, and with evaluation devices for processing the signals and determining the breath alcohol concentration. A simple construction is provided with high measuring accuracy by virtue of the pressure sensor (1) and the electrochemical sensor (2) being connected to a delta-sigma analogue-digital converter (8), which converts the sensor signals directly to digital measured values for determining the alcohol concentration. This renders the pre-amplifiers previously required superfluous, which presents a number of advantages.