Single-crystal EPR measurements have been performed on the triclinic form of [Fe(OEP)(NO)] (Ellison, M. K.; Scheidt, W. R. J. Am. Chem. Sec. 1997, 119, 7404) and on the isomorphous cobalt derivative [Co(OEP)(NO)] (Ellison, M. K.; Scheidt, W. R. Inorg. Chem. 1998, 37, 382) which has been doped with [Fe(OEP)(NO)]. Principal values of the g tensor determined at room temperature are g(max) = 2.106, g(mid) = 2.057, and g(min) = 2.015. The principal direction associated with the minimum g value lies 8 degrees from the Fe-N(NO) direction, 2 degrees from the normal to the heme plane, and 42 degrees from the N-O direction. The direction associated with the maximum g value lies 9 degrees from the normal to the Fe-N-O plane. The fact that the direction of g(min) is near the Fe-N(NO) direction is consistent with the dominant role of spin-orbit coupling at the iron atom in determining the g tensor and with the picture of the electronic structure of the compound from restricted calculations, which makes the half-filled orbital mostly d(z2) on the iron atom. The hyperfine tensor is nearly isotropic and was only resolved in the doped samples. Principal values of the A tensor determined at room temperature are 40.9, 49.7, and 42.7 MHz. Principal values of the g tensor determined from the doped samples at 77 K are g(max) = 2.110, g(mid) = 2.040, and g(min) = 2.012. Principal values of the A tensor are 42.5, 52.8, and 44.1 MHz at 77 K. The small change in g values with temperature is in contrast to the large temperature dependence on g values observed in samples of MbNO (Hori ct al. J. Biol. Chem. 1981, 256, 7849).