The partial oxidation of an aldehyde to a carboxylic acid has been studied over two single-phase oxidic catalysts and a mixture of both phases. One phase was mainly based on Mo and V (Mo-V-O-x); the other was a copper molybdate, CuMoO4. The experimental setup enabled the simultaneous determination of reaction kinetics and the oxygen activity of the catalyst under working conditions. The kinetic measurements were performed by monitoring the gas-phase composition along the length of a fixed bed of catalyst which was loaded into a sample port reactor. The reactor was treated as an isothermal plug-flow system. Over Mo-V-O-x the aldehyde could be oxidised to the corresponding acid with high activity and selectivity (maximum acid yield: 92 mol%). In contrast, the copper molybdate proved to be nearly inactive and unselective with respect to the acid production under the same reaction conditions, with the maximum yield of the acid always being below 2 mol%. Surprisingly, the selectivity towards the acid and as a consequence the acid yield could be significantly improved (maximum acid yield: 95 mol%) by mechanically mixing both oxidic phases. This demonstrates a synergism between the Mo-V-O-x and the copper molybdate in the case of the partial oxidation investigated. The simultaneous determination of the oxygen activity in the catalyst has been realised by use of a solid electrolyte potentiometry (SEP) cell, connected to the apparatus. The cell consists of an oxygen ion-conducting solid electrolyte (ZrO2 + 8.5 wt% Y2O3) coated with a platinum reference electrode on one side and with the catalytically active electrode on the other side. While the measuring electrode was in contact with the gas phase to be analysed, the reference electrode was always flushed with air. We used the same catalyst in the tubular reactor (typical acrylic acid catalyst Mo-V-Cu-O-x,) for all potentiometric measurements in order to obtain the, same gas-phase composition over the measuring electrode. The oxygen activity of the catalyst is derived from the measured potential difference between both electrodes. The results of the potentiometric measurements show that both single-phase catalysts, the Mo-V-O-x and the copper molybdate, are always in a reduced state under working conditions. As long as a significant amount of the aldehyde (>0.2 mol%) was present in the gas phase the Mo-V-O-x remained in a highly reduced state, reflecting the strong interaction between the aldehyde and the MoV-O-x. The less reduced state of Mo-V-O-x. in contact with a mixture containing the carboxylic acid instead of the aldehyde reflects the weaker interaction of Mo-V-O-x with the intermediate carboxylic acid. In contrast, the reduction of the copper molybdate catalyst was more pronounced in contact with a mixture containing the acid instead of the aldehyde. This indicates that the rate of the total oxidation of the acid is higher than the rate of its formation, which is the main reason for the low acid yields over the copper molybdate. The oxygen activity in Mo-V-Cu-O-x (1), which can be regarded as a mixture of Mo-V-Ox and CuMoO4, lay between the values for the single oxides, which can be interpreted by the transport of oxygen from the donor-phase CuMoO4 to the acceptor-phase MoV-O-x The transferred oxygen may act as a selective species, thus increasing the selectivity of the aldehyde oxidation with respect to the corresponding acid.