Zero-field splitting (ZFS) of three high-spin Co(I) (S = 1) clathrochelate complexes was determined by frequency-domain Fourier-transform THz-EPR (FD-FT THz-EPR). The following axial and rhombic ZFS values (D and E, respectively) were determined: [N(n-Bu)4]CoI(GmCl2)3(BPh)2 (1, D/hc = +16.43(1) cm-1, E/hc = 0.0(1) cm-1), [P(Me2N)4]CoI(GmCl2)3(BPh)2 (2, D/hc = +16.67(4) cm-1, E/hc = 0.0(1) cm-1), and [P(C6H5)4]CoI(GmCl2)3(BPh)2 (3, D/hc = +16.72(2) cm-1, E/hc = 0.24(3) cm-1). Complementary susceptibility χT measurements and quantum chemistry calculations on 1 revealed hard-axis-type magnetic anisotropy and allowed for a correlation of ZFS and the electronic structure. Increased rhombicity of 3 as compared to 1 and 2 was assigned to symmetry changes of the ligand structure induced by the change of the counterion. 1 and 3 exhibited temperature-dependent ZFS values. Possible reasons for this phenomenon, such as structural changes and weak chain-like intermolecular antiferromagnetic interactions, are discussed.