The auto-ignition of NH3/CH4/H-2/air mixtures constitutes a hazard that is of much concern in urea plants. In this study, the auto-ignition behaviour of NH3/CH4/H-2/air mixtures is investigated experimentally for pressures up to 7500 kPa. The experiments were carried out in a closed spherical vessel with a volume of 8 dm(3). The concentration and the pressure dependence of the auto-ignition temperature (AIT) were determined for four types of mixtures: NH3/air, NH3/CH4/air, NH3/H-2/air and NH3/CH4/H-2/air. The most ignitable NH3/air mixtures were situated between stoichiometry and the upper flammability limit. Small amounts of methane and hydrogen decrease the AIT of NH3/air mixtures to a large extent. The pressure dependence of the AIT could be correlated by a Semenov relationship. For the multi-fuel mixtures, a distinct deviation from the Semenov correlation was observed at the lowest temperatures. With respect to the explosion hazard in urea plants, the experimental results were used to assess realistic AIT values in the pool reactor and the ammonia scrubber, operating at a pressure of 15 000 kPa. (C) 2002 Elsevier Science B.V. All rights reserved.