At room temperature the absence of 29N2 when N2O reacts with a 30N2-saturated tungsten filament implies that only the nitrogen/oxygen bond is broken during the reaction. The catalysis of the nitrogen isotope exchange reaction 28N2+30N2= 229N2 by tungsten is completely inhibited at filament temperatures below 1600°K by the presence of N2O. Above this temperature the efficiency of catalysis rises rapidly and reaches its normal value at about 1800°K.