The effect of gas species on the degradation of gate oxides due to plasma exposure in reactive ion etching has been investigated. Gases tested were H-2, Ar, Xe, and O-2. The oxide degradation was evaluated by applying a constant-current stress and measuring the charge-to-breakdown Q(bd) of metal/oxide/silicon capacitors. It has been found that O-2 plasma significantly degrades the reliability of the gate oxide. Characterization of plasmas using a Langmuir probe has shown that O-2 gas tends to produce nonuniform plasma because of its electronegative nature, and thus enhances degradation of gate oxide.