The reactions of gas-phase atomic hydrogen (H(g)) and deuterium (D(g)) with the chemically modified Ir(111)-p(1 X 2)-O, and deuterium (D(a)) and hydrogen (H(a)) adatom precovered Ir(111) surfaces at 100 K have been studied using thermal desorption mass spectrometry. Although the Ir(111)-p(1 X 2)-O surface at a temperature of 100 K is passivated with respect to adsorption of gas-phase H-2 and D-2, the interaction of H(g) and D(g) with this surface at 100 K results in the subsequent desorption of water in thermal desorption spectra. These results suggest that while the dissociative chemisorption of molecular hydrogen on Ir(111) is precluded by the presence of the (1 X 2)-O oxygen overlayer, energetically "hot" H(g) reacts readily with this oxygen-modified surface. Moreover, a strong kinetic isotope effect has been observed in the interaction of H(g) and D(g) with D(a) and H(a) precovered Ir(111) surfaces at 100 K. We find that H(a) is more readily replaced by D(g) (abstraction cross section of sigma(D(g)) = 4.7 +/- 0.4 X 10(-16) cm(2)) than is D(a) by H(g) (sigma(H(g)) = 2.6+/-0.2 x 10(-16) cm(2)). These calculated cross sections assume a unity probability for reaction of H(g) and D(g) with the Ir(111)-p(1 X 2)-O surface. This observed isotopic difference in abstraction cross sections is consistent with the differences associated with the expected zero point energy of the transition state for the abstraction reaction and differences associated with the zero point energy between H and D adatoms.