The hydration numbers n of the deposited cobalt oxyhydroxide (CoOOH . nH(2)O) films were measured using an in-situ quartz crystal microbalance, and the effect of the La3+ ion on the hydration was studied. The hydration number of the deposited film was very high during the initial stage of the electrolysis (n > 10), but decreased to a constant value. The constant values of the hydration number were about 3 and 1 for CoOOH . nH(2)O films deposited in Co(CH3COO)(2) and in other solutions (Co(NO3)(2), CoCl2, CoSO4) respectively. The anion effect of CH3COO- on hydration was discussed. The incorporation of an La3+ ion increased the hydration number by about 5. Consequently, CoOOH . nH(2)O . La3+. 5H(2)O was proposed for the composition of the deposited cobalt oxyhydroxide film containing La3+ (atomic ratio La/Co = 1). The combination state of La3+ in this composition was also discussed.