The electronic spin transition in [Fe(btr)(2)(NCS)(2)] . H2O (btr = 4,4'-bis-1,2,4-triazole) is probed by longitudinal field muon spin relaxation measurements. The muSR spectra could be analyzed with two different components, corresponding to two muon sites. For both components, an increase in magnetic fluctuations is observed at the LS --> HS transition, i.e. crossing the spin transition temperature coming from low temperature, if a sufficiently high field is applied. Even below the transition temperature, magnetic fluctuations are still found. For one of the two union sites, muonium formation is clearly found below the spin-crossover temperature. (C) 2004 Elsevier B.V. All rights reserved.