New phosphoranyl radicals [(R3SiO)(3)POSiR’(3)](.) (R = R’ = Et, 1, R = iPr, R’ = Et, 2) were generated and studied by EPR, in both liquid and frozen n-hexane. 1 and 2 were shown to be very persistent (for 1, tau(1/2)similar to 50 min at 293 K), and their EPR isotropic spectrum is composed of a large doublet resulting from the phosphorus hyperfine coupling (Ap similar to 2800 MHz). The modulation of the anisotropic part of the hyperfine phosphorus coupling was shown to be the main relaxation mechanism contributing to the EPR line width. Its contribution was close to 98% at 230 K and decreased to 75% at 330 K. The contribution of the exchange coupling between two radicals was negligible at 230 K but reached 25% at 330 K. Radicals 1 and 2, strongly influenced the 1 of the solvent hydrogens, and in n-hexane at 273 K the efficiency factor of 2 was 0.55. Using this value and a model previously described for stable nitroxides, the maximum value of the low-field dynamic nuclear polarization factor of an n-hexane solution of 2 was found to be close to 23 x 10(3). This value is approximately 10 times higher than the value measured and calculated for the most efficient nitroxides currently used in low-field DNP experiments.