Alkalinity or buffering in anaerobic digesters may be chiefly attributed to bicarbonate and should be at least 1000 mg CaCO3/l for successful operation. A simple on-line instrument for the direct determination of bicarbonate concentration, especially for automatic control of anaerobic digesters, is described. The proposed method is based on a continuous measurement of the carbon dioxide flow rate evolved from a stream of sampled solution after saturation with gaseous CO2 and subsequent acidification with excess acid. This eliminates the need for a pH probe which is subject to fouling. The device was calibrated using sodium bicarbonate solutions (0.01-0.065 M) and the calibration showed an accuracy of ±5%. The device was tested on effluent from anaerobic digesters operating on ice-cream waste supplemented with sodium bicarbonate within a range of 0.02-0.04 mol/l. The results compared favourably with titrimetric measurements, extensively used to determine bicarbonate alkalinity with an accuracy of ±5%. The response of the instrument to variation in alkalinity concentration is sufficiently rapid for most industrial applications.