A miniature pCO2 electrode for possible CO2 measurement in a biological system was constructed and evaluated. The pCO2 electrode employed a miniature pH glass electrode and the Severinghaus configuration. The electrode was evaluated in both gas and liquid phases. A good linear relation existed between the measured electrode potential of the pCO2 electrode and the logarithm of the CO2 concentration at 20°C in both gas and liquid phases. The temperature dependence of the electrode over the temperature range of 20-60°C was evaluated. From 20 to 40°C, a change of CO2 of 0·15%/°C at a CO2 level of 9·6 mole% was observed. The response time, pCO2 and temperature-hysteresis effects, and long-term stability of the pCO2 electrode were also investigated and discussed.