Cobalt-benzene clusters, Co-n(benzene)(m), were synthesized by the reaction of laser-vaporized Co atoms with benzene vapor. The clusters exhibit magic-number behavior not only at m = n + 1 (n = 1-3) but also at (n-m) = (3-3), (4-4), (5-4), (6-4), (7-4), (8-5), and (9-6). Observed magic-number behavior and ionization energies are consistently explained by assuming the structure of the clusters to be multiple-decker sandwiches for n-m (m = n + 1, n = 1 - 3) and to be an n atom metal cluster coated by m benzene molecules for n-m (n greater than or equal to 4). In this paper, we will present the properties of Co-n(benzene)(m) clusters from the viewpoints of mass distribution, reactivity, and ionization energies. The structure and formation mechanism of the Con(benzene)m clusters will also be discussed.