The analysis methods described to date for 14 N electron spin echo envelope modulation (ESEEM) mostly deal with isotropic gand 14 N hyperfine coupling tensors. However, many cases of rhombic tensors are encountered. In the present report we present general equations for analyzing orientation-selective ESEEM and illustrate their use. (i) We present general equations for the nuclear interactions in an electron spin system where the EPR signal arises from an isolated Kramers doublet, then give the nuclear (electronnuclear double resonance) frequencies for I โซุโฌ 1 associated with such a system. (ii) These are incorporated into equations for single-crystal ESEEM amplitudes, which in turn are incorporated into general equations for the orientation-selective ESEEM that arises when the EPR envelope of a frozen-solution (powder) sample is determined by g anisotropy. (iii) This development is first used in the simplest limit of an isotropic g-tensor and leads to a more general picture of the response of the I โซุโฌ 1 modulation amplitude to variations in the nuclear hyperfine and quadrupole coupling constants, relative to the nuclear Zeeman interaction, than had been presented previously. We find that strong modulation occurs not only in the well-known regime where the "exact/ near cancellation" condition (A/2 ฯณ N ) is satisfied, but also when the nuclear hyperfine interaction is much larger than the nuclear Zeeman interaction (A/ N > 3) with A/K โซุโฌ 4 ฯณ 5. (iv) We then describe the orientation-selective 14 N ESEEM frequency-domain patterns (g vs frequency) in the presence of anisotropic (rhombic) hyperfine and electron Zeeman interactions for both coaxial and noncoaxial cases. We derive analytical solutions when the g-, hyperfine, and nuclear quadrupole tensors are coaxial. (v) The method is applied to the ESEEM of the nitrogenase MoFe protein (Av1) to determine the full hyperfine and nuclear quadrupole tensors of 14 N nuclei interacting with the S โซุโฌ 3 2 FeMo-cofactor (Fe 7 S 8 Mo: homocitrate).