Determinations of naturally occurring radiocarbon have been made with proportional counters filled with carbon dioxide at pressures up to 10 atmospheres. After chemical and radiochemical purification, the sample is converted into carbon dioxide for the counter filling. Extreme purity is required of the counter filling gas to prevent electron electron attachment by impurities. Procedures are described for producing carbon dioxide of sufficient purity to provide consistent and reproducible results. A counting efficiency of 100 percent is obtained for carbon‐14 betas contained in the active volume of the counter, giving a sample counting rate for the counter used of 45 cpm for contemporary material at a filling gas pressure of 10 atmospheres. With the present background of 13.5 cpm and a counting time limit of two days, dating may be extended to a maximum age limit of 42 000 years.