We have obtained a high concentration of P donor dopants in 6H-SiC enriched with 30Si and irradiated with thermal neutrons. It was established that annealing at a relatively low temperature of 1300 °C, i.e., 500–600 °C lower than that used for annealing SiC with the natural isotope composition after neutron-transmutation doping, gives rise to an electron paramagnetic resonance (EPR) signal corresponding to three different shallow P (sP) donors with large hyperfine interactions. The correlated changes of these sP centers in all the annealing experiments and the similarities to the spectra of shallow N donors demonstrate that these sites have shallow donor levels and a similar electronic structure and that they belong to different lattice sites: two quasicubic and hexagonal. The phosphorus at these three sites is suggested to occupy the C position. Simultaneously the low-temperature EPR signal from another set of P-related donor centers having a small, strongly anisotropic hyperfine interaction is observed. It is suggested that phosphorus in these centers occupies the Si position. Annealing at 1800 °C yields opposite changes in the concentrations of the two types of P-related donor centers: The EPR signals of sP centers disappear, while the intensity of the low-temperature EPR spectra of P donors considerably increases. Thus, the phosphorus at the C position is established to be unstable and annealing above 1700 °C causes P at the C site to move to the Si site. This process is vacancy mediated, as the temperature of this process is shown to depend on the thermal stability of intrinsic defects produced by neutron irradiation, whose concentration is proportional to the neut- ron irradiation dose.