Electron bombardment of ZnO powder leads to desorption of CO2. The presence of CO2 molecules at ZnO surfaces after chemisorption of oxygen was recently established from photodesorption experiments, leading to the conclusion that carbon impurities were active during oxygen chemisorption. Atomic hydrogen has earlier been found to chemisorb on ZnO single crystals, the adsorbed atoms acting as electron donors. We have tried to evaluate the possible influence of carbon impurities on this reaction with ZnO powder. It was found that adsorption of hydrogen atoms causes an increase of conductance, accompanied by desorption of CO and O. No significant desorption of CO2 was found.