The hydration numbers of electrodeposited cobalt oxy-hydroxide and manganese dioxide films containing lanthanide ions were measured using an in-situ quartz crystal microbalance. The hydration number was about 3 for the manganese dioxide film containing no lanthanide ion (MnO2 . 3H(2)O). The hydration number m per lanthanide ion was calculated by using the composition MnO2 . 3H(2)O . x(Ln(3+). mH(2)O). The hydration numbers per lanthanide ion were from about 5 to 11, and depended little on the kind of lanthanide ione. These values, except 5, are close to those for lanthanide ions in water (from 8 to 9), suggesting that the lanthanide ion will only adsorb on the surface of the deposited MnO2 particle during electrodeposition. However, the hydration numbers m per lanthanide ion in electrodeposited cobalt oxy-hydroxide, CoOOH . 3H(2)O . x(Ln(3+). mH(2)O), varied from about 2 to 10, and strongly depended on the kind of lanthanide ion. The hydration number decreased with increase in the lanthanide ion content in the cobalt oxy-hydroxide film. This result, as well as the hydration numbers smaller than those in water (8 and 9), suggest that the lanthanide ion, together with some water molecules, exists in the interlayers of the CoOOH structure.