Two tetraazaphosphorines, 1,4,5,6-tetrahydro-1,5-dimethyl-3,6-diphenyl-1,2,4,5,6-tetrazaphosphorine 6-oxide (4) and 1,4,5,6-tetrahydro- 1,5-dimethyl-3,3-diphenyl-1,2,4,5,3-tetrazaphosphorine 6-oxide (6), have been prepared and oxidized to form the corresponding stable "phosphaverdazyl" free radicals 5 and 7, respectively. Solution EPR spectra of both radicals were obtained, from which phosphorus hyperfine coupling values of 5.2 G for 5 and ca. 0.2 G for 7 were determined by spectral simulation. The very small value of the latter a(P) was rationalized on the basis of orbital symmetry arguments and infers a planar geometry of the heterocycle. In contrast, the larger a(P) for 5 is indicative of a nonplanar geometry at the phosphorus atom, consistent with structures of related verdazyl radicals. These results represent the first systematic foray into so-called "heteroveradazyl" radicals and demonstrate the utility of incorporating phosphorus as an additional probe for EPR studies.