The reactions of methanol on a size-selected nickel cluster ion, Ni-n(+) (n = 3-11), were investigated at collision energies less than 1.0 eV in a beam-gas geometry. Dominant reactions were methanol chemisorption, demethanation, and carbide formation. The absolute cross sections of these different reactions were measured and found to change dramatically with the cluster size; the demethanation proceeds preferentially on Ni-4(+), the carbide formation on Ni-7,8(+), and the chemisorption on Ni-6(+). A kinematic model explains the size-dependent characteristics that the chemisorption proceeds efficiently if the barrier height between the physisorbed and the chemisorbed states is low, and the demethanation and the carbide formation proceed otherwise. The rate of the carbide formation depends sensitively on an Ni-Ni distance of Ni-n(+) so that it proceeds only on Ni-7,8(+).