The electrochemical reactions occurring in the nickel oxide electrode system have been studied with the quartz crystal microbalance. Following cathodic deposition from nickel sulfate solutions, alpha-nickel hydroxide films were cycled in 1.0M alkali metal hydroxide solutions. The oxidation process produced frequency decreases in these solutions, indicating a corresponding increase in mass in the electrode layer. Upon subsequent reduction, a return to the initial frequency value was observed. These shifts ranged from a relatively small change for lithium hydroxide to progressively larger shifts as the cation size increased. It is clear from these results that alkali metal cations are being taken up into the electrode structure during oxidation. A mechanism based upon a net 1.7 electron oxidation has been proposed, involving predominant oxidation of nickel from the +2 to the +4 state as indicated by recent EXAFS studies. The mechanism is consistent with the quartz crystal microbalance data provided that alkali metal cations enter, and water molecules leave, the electrode during oxidation. beta-Nickel hydroxide films were prepared by contacting alpha-Ni(OH)(2) films with hot 8M KOH. Cycling these films in 1.0M alkali hydroxide solutions gave frequency changes which were positive for the lighter cations and negative for the heavier ones, in contrast to the results for alpha-Ni(OH)(2) films. Cation uptake during oxidation is also occurring in this case; however, a small but significant amount of water is also expelled, producing the observed frequency response. The mechanism in this case also involves some oxidation to the +4 state, as shown by XANES studies.