Continuous wave (CW) and pulse electron paramagnetic resonance (EPR), and electron nuclear double resonance (ENDOR) techniques were used to study frozen solutions of (oxyCo)TPP(L) (TPP = tetraphenylporphyrin, L = pyridine, 1-methylimidazole). By using a combination of CW EPR at X-, Q-, and W-band and Davies-ENDOR at Q-band, the g and cobalt hyperfine matrices and their corresponding principal axes were, for the first time, determined in detail. The variation of the g values as a function of temperature was followed by Q-band CW EPR. The existing ambiguity about the assignment of one of the g principal axes to the O-O bound direction was analyzed in detail. By studying the different proton interactions with Mims-ENDOR at X-band, a considerable solvent effect was found. With hyperfine sublevel correlation (HYSCORE) at X-band and two-pulse ESEEM (electron spin-echo envelope modulation) at S-band, the hyperfine and nuclear quadrupole interactions of the nitrogen nuclei of the axial ligand and the TPP ligand were determined. All the magnetic parameters were analyzed as a function of the geometrical and electronic structure of the complexes.