The magnetic behavior of polyradicals is usually understood on the basis of the spin polarization scheme displayed by the molecules. As far as conjugated hydrocarbon species are concerned, the spin distribution is rather well established and follows the sign alternation principle. Yet when a heteroatom is involved in the exchange pathway, the situation may become more delicate. This study concerns the involvement of a P-atom in the spin distribution and the intramolecular exchange interaction of nitronyl nitroxide radical-substituted triphenyl phosphine derivatives. The spin distribution of mono-, bi-, and tri-radical phosphine derivatives has been investigated by high-resolution fluid solution EPR and H-1 and P-31 MAS NMR spectroscopy. These techniques permitted one to establish that spin density is located at the phosphorus atom and showed that its sign depends on whether there is a P-lone pair or not. They also revealed the spin polarization scheme for the molecules. While these schemes are in line with ferromagnetic interaction between the radical units, the actual interactions are extremely weak as found by the magnetic studies. The experimental data are supported by DFT computations (UB3LYP/Lanl2DZ), which reproduce very well the effective spin distribution on these molecular systems except the sign inversion observed when going from the phosphine to its phosphine oxide counterpart. They also predict a very small high-spin/low-spin gap for these polyradicals.