The molecular building block of the first well-characterized purely organic crystalline ferromagnet is 4,4,5,5-tetramethyl-3-oxide-2(p-nitrophenyl)-2-imidazolin-1-yloxyl doublet radical. Among the diverse subjects of organic molecular crystalline ferromagnetism, the spin density distribution of the molecular building unit gives fundamental bases for understanding the underlying mechanism. In this work, an attempt has been made to determine experimentally the magnitudes and relative signs of the π-spin densities on the aromatic ring carbon sites (except the site 1) of 4,4,5,5-tetramethyl-3-oxide-2-phenyl-2-imidazolin-1-yloxyl doublet radical (abbreviated to 2- phenylnitronyl nitroxide) by liquid-phase ENDOR/TRIPLE spectroscopy. A series of partially deuterated 2-phenylnitronyl nitroxide compounds have given an unequivocal experimental identification of the ring protons: the spin densities of the ortho (2′,6′), meta (3′,5′), and para (4′) proton are ρortho= − 0.023, ρmeta= +0.013, and ρpara= -0.0207, respectively, and σiρi= -0.0407. The results show that 2-phenylnitronyl nitroxide undergoes negative spin polarization in the phenyl ring because the negative spins exceed the positive ones. The results have been compared with those obtained from polarized neutron diffraction measurements on the single crystal of 2-(p-nitrophenyl)- nitronyl nitroxide by Schweizer et al. The INDO calculation has been tested, indicating that the calculation gives an overestimation a great deal for the hyperfine coupling constants of the aromatic protons.