The millimeter/submillimeter-wave spectrum of the CoCl radical (X (3)Phi(i)) has been recorded using direct absorption techniques in the frequency range 340-510 GHz. This work is the first pure rotational study of this molecule. The radical was created by the reaction of Cl-2 with cobalt vapor. Rotational transitions arising from the Omega=4, 3, and 2 spin-orbit components of (CoCl)-Cl-35 have been measured, all of which exhibit hyperfine splittings due to the Co-59 nucleus (I=7/2). Transitions arising from the (CoCl)-Cl-37 species were also recorded, as well as those originating in the v=1, 2, 3, and 4 vibrational states of both isotopomers. The spin-orbit pattern exhibited by the molecule is unusual, with the Omega=3 component significantly shifted relative to the other spin components. In addition, the regular octet hyperfine splittings become distorted above a certain J value for the Omega=3 transitions only. These effects suggest that the molecule is highly perturbed in its ground state, most likely a result of second-order spin-orbit mixing with a nearby isoconfigurational (1)Phi(3) state. The complete data set for (CoCl)-Cl-35 and (CoCl)-Cl-37 were fit successfully with a case (a) Hamiltonian but required a large negative spin-spin constant of lambda=-7196 GHz and higher order centrifugal distortion corrections to the rotational, spin-orbit, spin-spin, and hyperfine terms. The value of the spin-spin constant suggests that the Omega=3 component is shifted to higher energy and lies near the Omega=2 sublevel. The hyperfine parameters are consistent with a delta(3)pi(3) electron configuration and indicate that CoCl is more covalent than CoF. (C) 2004 American Institute of Physics.