The dimethoxyphoshinyl-phenyl and C-13 enriched diisopropoxyphosphinyl-phenyl iminoxy radicals have been prepared by reaction of corresponding oximes with AgO. The magnetic properties of the radicals have been studied by a combined experimental and theoretical approach. The results of B3LYP and B1LYP computations are in excellent agreement with the EPR parameters determined on the basis of simulation of the experimental spectra. A difference observed in experimental hyperfine coupling with the iminoxy C-13 nucleus on going from Z to E isomer of the radical, 16.9 and 24.2 G, respectively, is discussed on the basis of calculated spin density distribution and natural bond orbitals analysis.