Experimental studies of the catalytic ignition of hydrogen-oxygen were performed. A spherical platinum of 1.5mm in diameter was used for catalyst and the atmospheric pressure was 0.1MPa. The parameters were equivalence ratio and dilution ratio. Dilution ratio is mole fraction of nitrogen. The surface temperature was measured with thermocouple and the time histories of surface temperature show an inflection point. The catalytic ignition temperature is defined as the temperature at the inflection point. The catalytic ignition temperature was increased with dilution ratio when equivalence ratio was constant. The effect of dilution ratio was explained well with adsorption rate of reactants. If equivalence ratio was less than 0.1, the catalytic ignition temperature was decreased with equivalence ratio increase, and was increased with equivalence ratio if equivalence ratio was more than 0.2. The desorption of adsorbed hydrogen should be considered to explain the effect of equivalence ratio.