Structural and energetic properties of the adducts formed by adsorbing methanol onto size-selected gold clusters are investigated by infrared photodissociation of trapped Au-n(+)(CH3OH)(m), n=1-10,15 and m=1-3. The excitation of vibrational modes of methanol leads to the desorption of neutral molecules which is monitored by time-of-flight mass spectrometry. Spectra are obtained by measuring the fragment ion intensity as a function of photon energy. The C-O stretching vibration of adsorbed methanol changes discontinuously with cluster size. By comparison with Car-Parrinello calculations this change is traced back to the dimensionality of the gold clusters. The number of photons necessary for the desorption of methanol molecules provides an estimate of the respective separation energies.