The thermal spin transition which occurs reversibly in [Fe(phen)(2)(NCS)(2)] (polymorph I) around T-1/2 similar to177 K upon cooling and warming has been investigated by muon spin relaxation (muetaSR) measurements over the temperature range (similar to10-280 K). The depolarization curves in zero and longitudinal fields (50-2000 Oe) are well described by two Lorentzian lines that represent fast and slow components in the decay curves and a nonrelaxing component. Evidence of the scope to track the spin transition occurring in [Fe(phen)(2)(NCS)(2)] (polymorph I) is provided via the temperature dependence of the zero-field initial asymmetry parameter of the fast component. Muonium-substituted radicals delocalized on the phenanthroline ring (hyperfine coupling constant A similar to500 MHz), as well as diamagnetic muonic species, have been identified on the basis of the applied field muSR experiments.