The spin state of the major isomer of Gd@C-82 (Gd@C-82-I) was determined by the combined use of a high-field electron spin resonance (ESR) spectrometer (W-band) and a conventional spectrometer (X-band). The ground spin state of Gd@C-82-I is characterized by an integer spin quantum number of S = 3. The spin (S = 1/2) on the pi orbital of the fullerene cage is coupled with the octet spin (S = 7/2) of the encapsulated gadolinium. in an antiferromagnetic manner with the exchange coupling constant of J = -1.8 cm(-1). In a condensed solution and solid powder, the Gd@C-82-I molecules easily formed dimers. The radical spins on the cage are canceled upon dimerization, and the resultant spin state of the dimer is described by two independent octet (S = 7/2) spins of the gadolinium. The X- and W-band ESR spectra for Gd@C-82-I are completely reproduced by an eigenfield method simulation, and ESR parameters for the g tensor, zero-field splitting tensor, and the intramolecular exchange coupling constant, J, have been determined. From the temperature dependence of the W-band ESR spectrum, the sign of the zero-field splitting constant D has been determined to be positive.