A large pilot plant was constructed to study the upper flammability limit of ethene-air-nitrogen mixtures under conditions of flow. The gas mixtures flowed through an explosion tube with a length of 3.0 m and a diameter of 21 mm. An electrically heated wire was used as ignition source. Experiments were performed at pressures of 5 and 10 bar, with gas temperatures between 25 and 300 °C and with the wire fixed horizontally and vertically. Three different phenomena are observed: negligible reaction, local reaction, and explosion. The negligible reaction region occurs at power supply rates to the wire below a critical value. Above this critical value either a local reaction or an explosion occurs. The critical oxygen concentration which separates the local reaction and explosion regimes depends on the experimental conditions: gas composition, pressure, temperature, wire size and orientation, and gas velocity. An increase in pressure increases the upper flammability limit. Also an increase in temperature causes an increase in the upper flammability limit and the results can be explained by assuming a constant flame temperature. Moreover, the upper flammability limit is influenced by the gas velocity. Under conditions of flow the explosion region becomes smaller, it shifts to higher oxygen concentrations. In practise this may mean that partial oxidation reactions can safely be operated at higher oxygen concentrations, provided gas flow rates are kept high.