Previously described ion-selective membrane electrodes useful for determination of total carbon dioxide in biological fluids in automated analyzers show interference from several sources--e.g., fatty acids, keto acids, salicylate, heparin. Judicious selection of membrane components has produced a membrane with superior performance characteristics: short conditioning time, long lifetime in storage, rapid and stable response, low drift, and significantly less susceptibility to interference than other electrodes thus far reported. Samples can be analyzed at 240 samples or more per hour and results correlate well with those by, e.g., the Technicon SMAC II method for total carbon dioxide. Mounted on the Technicon RA-1000, the carbon dioxide sensor is arranged in tandem with sodium and potassium ion-selective electrodes. When all three species are measured in the same buffered sample stream, the sampling rate thus becomes 720 tests per hour, done on 25-microL samples.