In a previous paper we described the reduction of O-2 and CO2, in the presence of each other, at unshielded microdisc electrodes in an aprotic solvent with minimal cross-interference between the competing O-2 and CO2 reduction reactions. In this new work, we describe a practical Clark-type membrane-covered sensor for the simultaneous measurement of O-2 and CO2. The sensor comprises a gold disc microelectrode, housed in a PTFE holder and covered with a PTFE membrane, with dimethylsulphoxide (DMSO) as the solvent and with a silver quasi-reference electrode. The polarizing voltage is swept from 0 to -2.4 V vs. Ag at a typical sweep rate of 0.25 or 0.5 V s(-1) and the O-2 and CO2 concentrations are determined from the limiting currents of the well separated O-2 and CO2 reduction waves. We demonstrate that the addition of up to 10% v/v H2O to the solvent does not appear to compromise the ability of the sensor to analyse O-2 and CO2 simultaneously.