Abstract: A method for monitoring the toxic constituent of an ambient gas e.g. sulphur dioxide in air involves bringing the gas into contact with a solution containing one component in which the gas is soluble and an indicator having optical transmission characteristics responsive to the quantity of the constituent in solution. An electric current is then passed through a second component which is electrolysed to neutralise the solution and restore the transmission characteristics of the indicator to a datum, the quantity of current passed being representative of a change in transmission characteristics from the datum and therefore in the case of the concentration of the toxic constituent where sulphur dioxide in air is to be monitored the air is bubbled through water in which the sulphur dioxide dissolves. The water contains hydrogen peroxide to convert the sulphur dioxide in solution to sulphuric acid which is effective to change the color or optical density of a pH responsive indicator such as methyl orange.