The effect of relative particle size on the high-spin (HS, 5T2) α low-spin (LS, 1A1) equilibrium in carbon- or MgO-supported Fe(ppi)2(NCS)2 (ppi = N-phenyl-2-pyridinaldimine) has been studied by using Mössbauer spectroscopy. As the iron-complex loading is gradually decreased, the room-temperature Mössbauer spectra show that the fraction of HS is decreased and a LS species of increasing intensity appears. When the iron loading is less than 1.8 wt% on carbon or MgO, the room-temperature Mössbauer spectrum consists only of the quadrupole doublet corresponding to the LS species. This phenomenon has not been reported in any other spin-crossover study.