Sensitivity analysis has been used to study the gas-phase chemistry near a hot catalytic wall. The gas mixture was H-2/O-2 in different proportions and the studied pressure range was 1 to 10(5) Pa. The reaction mechanism for a polycrystalline Pt surface was used to model the catalyst surface, at a temperature of 1300 K. It is found that the chemistry of water production is relatively simple. For pressures of 1 to 10(3) Pa it is the transport of reactants to the surface, via sticking, which determines the water concentration in the gas-phase. For these low pressures almost all water is produced on the surface since there is no combustion zone in the gas. The water which is produced on the surface will desorb and then escape from the reactor without any further reactions. The situation is much more complex when an intermediate species, such as the O atom, is considered. O atoms in the gas-phase are prone to react with other species. This gives a complicated interaction between the surface and the gas-phase chemistry. The consumption of O atoms in the reaction O + H-2 reversible arrow OH + H is a major inhibitor for the O-atom concentration. It is found that the O atoms which are created in the gas originate mainly from desorbed OH, since the reaction H-2 + OH reversible arrow H2O + H yields H atoms which contribute to O-atom production in the reaction H + O-2 reversible arrow OH + O. For the higher pressure range 10(4) to 10(5) Pa, there is a flame front at a certain distance from the surface, and the surface reactions are far less important than the gas-phase reactions. The reactions differ considerably in importance depending on whether the gas is fresh or burnt.