We present an electron spin resonance (ESR) study at excitation frequencies of 9.4 and 222.4 GHz of powders and single crystals of a Prussian blue analog (PBA), RbMn[Fe(CN)(6)]center dot H2O in which Fe and Mn undergoes a charge-transfer transition between 175 and 300 K. The ESR of PBA powders, also reported by Pregelj et al. [J. Magn. Magn. Mater. 316, e680 (2007)], is assigned to cubic magnetic clusters of Mn2+ ions surrounding Fe (CN)(6) vacancies. The clusters are well isolated from the bulk and are superparamagnetic below 50 K. In single crystals, various defects with lower symmetry are also observed. Spin-lattice relaxation broadens the bulk ESR beyond observability. This strong spin relaxation is unexpected above the charge-transfer transition and is attributed to a mixing of the Mn3+-Fe2+ state into the prevalent Mn2+-Fe3+ state.