The effect of ballistic electron injection into aqueous solutions has been studied by nanocrystalline silicon (nc-Si) planar cold cathode. The present article shows that the nc-Si ballistic emitter operates in both pure water and a dilute H2SO4 solution, and that hydrogen gas is generated with no by-product like oxygen. The hydrogen evolution rate in H2SO4 solution was higher than that in water as expected. In water, the pH value shows a significant increase toward the alkaline side during the device operation without using any counterelectrode. These results support the model that energetic electrons emitted from the device into solutions directly reduce H+ ions at the interface. The ballistic emitter is useful in solutions not only for hydrogen generation but also for control of pH level and related electrochemical properties. (C) 2008 American Vacuum Society.