The Ph4P+ salt of the tetrahedral complex [Co(SPh)(4)](2-), possessing an S = 3/2 ground state with an axial zero-field splitting of D = -70 cm(-1), displays single-molecule magnet behavior in the absence of an applied magnetic field. At very low temperatures, ac magnetic susceptibility data show the magnetic relaxation time, tau, to be temperature-independent, while above 2.5 K thermally activated Arrhenius behavior is apparent with U-eff 21(1) cm(-1) and tau(0) = 1.0(3) X 10(-7) s. Under an applied field of 1 kOe, tau more closely approximates Arrhenius behavior over the entire temperature range. Upon dilution of the complex within a matrix of the isomorphous compound (Ph4P)(2)[Zn(SPh)(4)], ac susceptibility data reveal the molecular nature of the slow magnetic relaxation and indicate that the quantum tunneling pathway observed at low temperatures is likely mediated by intermolecular dipolar interactions.