Isotropic ESR spectra of the isooctyl radical, (CH3)3CCH2C(CH3)2, have been observed in the irradiated thiourea-isooctane complex in the temperature range between 77 and 318 K. The structure of the isooctyl radical has been determined and the temperature dependence of the hyperfine splittings was explained by the simple formulation developed for torsional oscillation. The frequency of torsional oscillation of-C(CH3)2 group has been estimated to be 58 ± 2 cm-1. © 1985 American Chemical Society.