A sensor for hydrogen sulphide is described. It is based on the catalytic effect of the gas on the iodine-azide reaction. An iodine-azide solution is exuded from a sintered-glass ball (10mm diameter) at a flow-rate of 3.30 ml min whilst its potential is monitored by two platinum electrodes. one inside and the other outside the ball. Sample is carried in a gas at a flow-rate of 350 ml min . Carbon disulphide, methyl mercaptan or sulphur dioxide in amounts up to a hundred times that of the hydrogen sulphide do not interfere. The detection limit is 5 ng and the determination limit is 8 ng, with a relative standard deviation of 10%.