An a.c. coupled quench circuit is used to quench counters containing non-selfquenching carbon dioxide-carbon disulphide mixtures for the measurement of radioactive carbon as carbon dioxide. The counters operate reliably at quite high counting rates and continuous discharges do not occur. Plateaux 200 V or more long with slopes of about 2 to 4% per 100 V are obtained. Accurate corrections for resolving time loss can be made for counting rates up to at least 15 000 counts/min.