We have determined, by polarized neutron diffraction, the spin density of two nitrophenyl nitroxides : the para-nitrophenyl nitronyl nitroxide (p-NPNN), which is a ferromagnet at low temperature, and the meta-nitrophenyl imino nitroxide (m-NPIN), which exhibits antiferromagnetic coupling. Several methods were used to reconstruct the spin density distribution, including the maximum entropy method. Our data provide the basis for a better knowledge of the electronic configuration. The nodes of the SOMO are directly observed and the spin polarization effects which give rise to a negative spin density on several atomic sites is discussed.