We have investigated the magnetic properties of iron mixed-valence complexes, (n-CnH2n+1)(4)N[(FeII-FeIII)(dto)(3)] (dto = C2O2S2, n = 3, 5), in which not only a ferromagnetic transition but also a novel charge transfer phase transition (CTPT) take place [1]. This CTPT can be observed under ambient pressure for n = 3, while it appears abruptly above 0.5 GPa for n = 5 [2]. Recently, we have measured the muon spin relaxation (mu SR) for the CTPT of n = 3, which revealed the dynamical process of electron-transfer between Fe-II and Fe-III and its frequency was estimated at about 0.1 MHz [3]. To investigate the pressure induced CTPT for n = 5, we carried out the mu SR measurement for n = 5 at 150 K between 0.30 and 0.64 GPa with the He-4 gas-operated pressure system. The asymmetry of the muon spin relaxation for n = 5 with Cu-Be pressure cell was almost constant up to 0.55 GPa, while it rapidly decreased with increasing pressure above 0.60 GPa. This result shows that the applied pressure causes the spin fluctuation due to the CTPT, which induces the decrease of the asymmetry of muon spin relaxation. This experiment can correctly decide the phase transition pressure from the absence to the appearance of the CTPT for n = 5. (C) 2011 Elsevier Ltd. All rights reserved.