Axialgtensors and α-hydrogen hyperfine tensors for frozen (CH3)3MĊH2radicals (M = C, Si, Ge, Sn and Pb) have been obtained by computer fit to first derivative E.P.R. spectra. Rotation about the carbon-metal bond is found to be rapid at 77°kexcept for the first in the series, neopentyl radical, which required 140°kfor coalescence of the superhyperfine structure from theγprotons. Theg‖values are constant for the series at 2·0029 (±0·0004), while theg⊥vales are 2·0024, 2·0023, 2·0020, 1·9997 and 1·9938 (±0·0002), in order, from M = carbon to lead. The α-hydrogen hyperfine splittings are essentially constant at 21 oe for the series. Previously reportedγ-hydrogen hyperfine splittings are reflected in the observed line widths. Their values suggest a change in the sign of the spin density onγ-H in the region of germanium. The contact hyperfine splitting of207Pb was found to be 158 oe. This measurement makes available the hyperfine splittings of α atoms in planar π radicals over the entire length of the periodic table for the first time. It is found that a spin-polarization coefficient can be defined which varies by less than a factor of 2 over this range. This coefficient may prove useful in predicting spin densities or hyperfine splittings for heavy atom-substituted radicals. Calculation ofgvalues based on vacant metaldorbitals predicts the observed trends. These calculations allow estimates of the metald-orbital participation in the ground state of the π radical. This mixing coefficient is approximately 1 per cent and is essentially constant from silicon to lead.