Chemisorption of a methanol molecule onto a size-selected copper cluster ion, Cu-n(+)(n = 2-10), and subsequent reactions were investigated in a gas-beam geometry at a collision energy less than 2 eV in an apparatus based on a tandem-type mass spectrometer. Mass spectra of the product ions show that the following two reactions occur after chemisorption: dominant formation of Cu-n-1(+)(H)(OH) (H(OH) formation) in the size range of 4-5 and that of CunO+ (demethanation) in the size range of 6-8 in addition to only chemisorption in the size range larger than 9. Absolute cross sections for the chemisorption, the H(OH) formation, and the demethanation processes were measured as functions of cluster size and collision energy. Optimized structures of bare copper cluster ions, reaction intermediates, and products were calculated by use of a hybrid method (B3LYP) consisting of the molecular orbital and the density functional methods. The origin of the sizedependent reactivity was explained as the structural change of cluster, two-dimensional to three-dimensional structures.