Cobalt-benzene clusters, Co,,(benzene),,,, were synthesized by the reaction of laser-vaporized C o 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 iz-m ( n 2 4). In this paper, we will present the properties of Co,,(benzene),,, clusters from the viewpoints of mass distribution, reactivity, and ionization energies. The structure and formation mechanism of the Co,,(benzene),,, clusters will also be discussed.