To obtain information on the electronic state of iron ion in various hemoproteins (myoglobin, hemoglobin, Rhodospirillum rubrum hemoprotein (RHP) and cytochrome c (Cyt. c)), the magnetic susceptibilities of these substances were measured from room temperature down to 4.2° K. After plotting the experimental susceptibility data against the reciprocal of temperature, the diamagnetic part of the magnetic susceptibility could be easily determined by extrapolating the χ-1/ T curve to the point where 1/ T was zero. The temperature dependence of the effective number of Bohr magnetons was calculated from the paramagnetic part, which was obtained by substracting the diamagnetic part from the total magnetic susceptibility. The following compounds were of high-spin type: Mb(Fe 3+) pH = 6,Mb(Fe 2+) pH = 6, Mb(Fe 3+)F, Hb(Fe 3+) pH = 6, Hb(Fe 2+) pH = 6, RHP(Fe 3+) pH = 6, RHP(Fe 3+) pH = 11, RHP(Fe 2+) pH = 6, RHP(Fe 2+) pH = II , catalase (Fe 3+) pH = 6 and catalase (Fe 3+)F. From the temperature dependence of n eff, the value of D (where H= DS z 2) of Mb(Fe 3+) pH = 6 was estimated to be 10 cm −1 which was in good agreement with the value obtained from the anisotropy of the susceptibility. Mb(Fe 3+)CN, Cyt. c (Fe 3+) pH = 6, Cyt c (Fe 3+) pH = 11, Cyt. c (Fe 2+) pH = 6, Cyt. c (Fe 2+) pH = 11, catalase (Fe 3+)CN and catalase (Fe 3+)N 3 were of low-spin type and n eff for Mb(Fe 3+) pH = 9 had a medium value, between the high- and low-spin types.