Hydrogen effects on the electrical and optical properties of p-i-n ZnO light-emitting diodes (LEDs) were investigated. There were no diode characteristics or light emissions observed unless the structures were annealed at 350 C after fabrication. Annealed diodes showed bandedge electroluminescence (385 nm) and a broad defect band with a peak at 930 nm at room temperature. A hydrogen plasma was used to deliberately introduce hydrogen into the annealed p-i-n ZnO LEDs to verify its effects on diode characteristics. With hydrogen plasma treatment, the diode reverse-bias leakage current increased and both the turn-on voltage and the intensity of the electroluminescence decreased. Moreover, the effects of moisture, water, and phosphoric acid solution on the annealed diode characteristics were also investigated and significant degradation of electrical and optical properties were observed in all cases. These results point to the sensitivity of ZnO LEDs to processing conditions and measurement ambient. (C) 2008 The Electrochemical Society.