The physical state changes of amorphous FeOOH incorporating aniline derivatives [FeOOH(AD)(1/4)] in the course of discharge and charge cycling were studied using electron spin resonance (ESR). Two peaks were observed for the ESR signal of FeOOH(AD)(1/4), which were related to the radicals in the aniline derivative and electron spin of the iron ions. The electron spin concentrations corresponding to these peaks were estimated from the ESR signals. In the first discharge and charge cycle, the electron spin concentration for iron ions changed irreversibly. However, the change in the electron spin concentration for iron ions was reversible for the second and subsequent discharge and charge cycles. These findings clearly show that the irreversible physical property changes for FeOOH(AD)(1/4) take place during the first discharge. The electron spin concentration for the aniline derivatives also changed with the oxidation or reduction of iron ions in FeOOH(AD)(1/4). This result suggests the presence of an electronic interaction between aniline derivatives and iron ions in FeOOH(AD)(1/4).