Abstract: A calibrating device for a breath alcohol measuring instrument which includes a passage for the breathing air which has a sensor and a collector arranged therein with a discharge to atmosphere from the collector comprises a connecting circuit having valve means for selectively connecting the circuit through the passage. The connecting circuit has a member with a chamber therein which has a piston wall member movable in the chamber between two end positions and spring means biasing it toward one end position. A calibrating gas supply and a sensor are selectively connectable to the chamber on one side of the movable piston wall member. Control means are connected to the chamber and the sensor and to the valve means for selectively connecting either the calibrating gas supply or the sensor to the chamber and to the passage for the breathing air in the measuring instrument.