An instrument is described which records conductimetrically the concentration of sulphur dioxide in the range 2-50 parts per million and carbon dioxide in the range 0.005-0.5% v/v in air over a 5 minute sampling period with the use of Nash impinger cells with coefficients of variation of 5%. Higher or lower concentrations can be determined using 2.5 or 10 minutes sampling times. The effect of operating conditions is discussed and full construction and operating details are given.