Using the methods of UV-, VIS- and near-IR spectrometry, potentiometry, static magnetic susceptibility and cyclic voltammetry, an iron(II) complex with tris(pyrazol-1-yl)methane (L) having the composition of [FeL2]SO4 is studied in aqueous solution. In the solid state this compound exhibits spin crossover 1A1 ↔ 5T2. The protonation constants of L (log K1H=2.53, log K2H=1.80) and stability constants of the complex (log β(FeL2+)≈ 4, log β(FeL2 2+)=8.62) at t = 25°C, I=0.05 M have been determined. The studies on μeff(T) of [FeL2]SO4 in solution in the temperature range from 280 to 355 K have shown that the value of μeff exhibits an increase with increasing temperature. This indicates that with increasing temperature a transition occurs from the low spin (LS) state of the complex to the high spin (HS) state thereof.