The effects of hydrogen plasma treatment on high-quality bulk n-ZnO crystals were studied. It is shown that after plasma exposure at 200 °C for 0.5 h the hydrogen penetrates into the material down to about 20 μm and shows concentrations close to 1017 cm-3 in that region. The incorporation of this hydrogen coincides with an increase in the shallow donor concentration to about the same level as the concentration of hydrogen. In contrast to that in most other semiconductor materials, hydrogen plasma treatment of ZnO is shown to increase the concentration of the already existing electron and hole traps and to introduce electron traps near 0.55 eV, earlier observed in proton irradiated samples. The effect is at least partially due to the surface damage caused by plasma exposure. Despite this increase in the density of deep traps, the luminescence intensity in the near band-edge region is shown to increase down to the depth corresponding to the hydrogen penetration depth in the material. © 2003 American Institute of Physics.