A novel, totally electrochemical pH-stat has been constructed and evaluated. The system has a minimum time constant of 2–3 s at which no overshoot or ringing is observed and has a current efficiency of nearly 100% over a wide range of conditions. The applicability of the device to the measurement of reaction rates was tested by determining the hydrolysis rate of a reasonably rapid reaction, namely the hydrolysis of tert-butyl chloride in water at pH 7 (k = 0.041 ± 0.0007 s−1). It was further tested by assaying the activity of the enzyme penicillinase over the range 0.4 to 26 IU, by measurement of the enzyme's pH profile and pH optimum (7.2) and by determining the Michaells constant of the substrate penicillin G (88 μM). The slat may be used as a controlled current coulometric acid-base analyzer. Good precision and coulometric current efficiency were obtained when mineral acid, in amounts from 0.02 to 5 μmol was rapidly added to a 50-ml electrolysis vessel or when the acid was added slowly from a constant rate buret at rates ranging from 0.1 to 22 μmol/min. Amounts of penicillin G ranging from 0.15 to 3.2 μmol were also measured by direct enzymic hydrolysis in the electrolysis vessel. In general, the device is sufficiently fast to permit approximately 100 determinations of acid or base, or about 20 rate determinations per hour.