A novel combinational (\mathrm{pH}-\boldsymbol{P}{\mathrm{CO}{2}}) microelectrode based on a neutral carrier hydrogen ion exchanger is described. It is easy to fabricate and allows (\mathrm{pH}) and (\boldsymbol{P}{\mathbf{C O}{\mathbf{2}}}) to be measured simultaneously. The microelectrode has a 5- (\mu \mathrm{m}) tip. The (\boldsymbol{P}{\mathrm{CO}{3}}) microelectrode exhibits a linear response in the range (1.75 \times 10^{-5}-10^{-2} \mathrm{~mol} /) liter with a Nernstian slope of (57.0 \mathrm{mV} /) decade at (25^{\circ} \mathrm{C}). The detection limit is (10^{-6} \mathrm{~mol} / \mathrm{liter}). The (\mathrm{pH}) microelectrode (\mathrm{ex}-) hibits a linear response in the range (\mathrm{pH}) 4-12 with a Nernstian slope of (60.0 \mathrm{mV} /) decade at (25^{\circ} \mathrm{C}). The (95 %) steady-state response time of the (\boldsymbol{P}{\mathbf{C O}{2}}) microelectrode is about (2 \mathrm{~min}), while it is about (10 \mathrm{~s}) for (\mathrm{pH}) microelectrode. The electromotive force drift is (4.3 \mathrm{mV} / \mathrm{h}\left(\boldsymbol{P}{\mathrm{CO}{2}}\right)) and (2.6 \mathrm{mV} / \mathrm{h}(\mathrm{pH})), respectively. The lifetime is 3 to 4 days. The microelectrode can measure (\mathrm{pH}) and (\boldsymbol{P}{\mathrm{CO}{2}}) in body fluids simultaneously with satisfactory results. It is also a good basic electrode for enzyme microelectrolysis. & 1993 Academic Press, Inc.