Abstract Estimation of pH, P CO 2 and total CO 2 in serum-based samples by laboratories in New York State was evaluated. Participant laboratories (122) showed average coefficients of variation of 8.8% for pH, 13.3% for P CO 2 and 15.0% for total CO 2 measurements. Coefficient of variation for measurement of pH in aqueous buffers was < 4%. A slightly improved performance in the estimation of P CO 2 was noted for laboratories using quality control sera. Participant data, grouped by manufacturer of equipment used, revealed neither mean values nor standard deviations substantially different from overall performance. A direct correlation is shown between weekly workload and laboratory performance in this area of chemical testing.