The influence of CO and CO2 on hydrogen-air detonations in round tubes was studied numerically and experimentally. The quasi-one-dimensional Zel＇dovich model of detonation limits and a detailed kinetic mechanism of H-2/CO oxidation (72 elementary reactions, 13 species) were used to estimate the detonation sensitivity to kinetic parameters and tube diameter. The numerical results were verified by comparison with various experimental data available for H-2/air detonations and with our own measurements in 41-, 24-, and 14-mm i tubes. For H-2/CO/air and H-2/CO/CO2/air detonations, we calculated and measured the concentration limits in a 24 mm fixed-diameter tube at initial pressure 0.1 MPa and temperature 298 K. It is shown that the limiting concentration of hydrogen in lean hydrogen/air mixtures decreases from 15.5 to 2.4 vol% with addition of 20 vol% carbon monoxide. Dilution of the mixture with CO2 results in a substantial shrinkage of the detonability domain. The numerical predictions are in reasonable agreement with the experimental results.