Six pseudotetrahedral cobalt(II) complexes of the type [CoL2Cl2], with L = heterocyclic N-donor ligand, have been studied in parallel by magnetometry, and high-frequency and -field electron paramagnetic resonance (HFEPR). HFEPR powder spectra were recorded in a 50 GHz < nu < 700 GHz range in a 17 T superconducting and 25 T resistive magnet, which allowed constructing of resonance field vs frequency diagrams from which the fitting procedure yielded the S = 3/2 spin ground state Hamiltonian parameters. The sign of the axial anisotropy parameter D was determined unambiguously; the values range between -8 and +11 cm(-1) for the given series of complexes. These data agree well with magnetometric analysis. Finally, quantum chemical ab initio calculations were performed on the whole series of complexes to probe the relationship between the magnetic anisotropy, electronic, and geometric structure.