The magnetic properties of pseudotetrahedral Co(II) complexes spawned intense interest after (PPh4)(2)[Co(SPh)(4)] was shown to be the first mononuclear transition-metal complex displaying slow relaxation of the magnetization in the absence of a direct current magnetic field. However, there are differing reports on its fundamental magnetic spin Hamiltonian (SH) parameters, which arise from inherent experimental challenges in detecting large zero-field splittings. There are also remarkable changes in the SH parameters of [Co(SPh)(4)](2-) upon structural variations, depending on the counterion and crystallization conditions. In this work, four complementary experimental techniques are utilized to unambiguously determine the SH parameters for two different salts of [Co(SPh)(4)](2-): (PPh4)(2)[Co(SPh)(4)] (1) and (NEt4)(2)[Co(SPh)(4)] (2). The characterization methods employed include SQUID magnetometry, high-field/high-frequency electron paramagnetic resonance (HF-EPR), variable-field variable-temperature magnetic circular dichroism (VTVH-MCD), and frequency domain Fourier transform THz-EPR (FD-FT THz-EPR). Notably, the paramagnetic Co(II) complex [Co(SPh)(4)](2-) shows strong axial magnetic anisotropy in 1, with D = 55(1) cm (-1) and E/D = 0.90(3), but rhombic anisotropy is seen for 2, with D = +11(1) cm (-1) and E/D = 0.18(3). Multireference ab initio CASSCF/NEVPT2 calculations enable interpretation of the remarkable variation of D and its dependence on the electronic structure and geometry.