Photo-induced reactions in the metal cation-molecule complexes Mg+(CH3F)(n) have been studied as a function of the number of solvent molecules. While a photoreaction of the singly solvated complex Mg+-FCH3 yields exclusively CH3+, excitation of larger complexes Mg+(FCH3)(2-4) produces predominantly bare and solvated MgF+. Photo-induced evaporation of the larger complexes was also observed, although with much lower yields. Possible mechanisms are suggested to interpret the abrupt change in the photoreaction patterns with an increasing complex size. The action spectra of all the complexes are discussed based on the transitions P-2 <--S-2 centered on the Mg+ ion but perturbed by the presence of the FCH3 molecules. Quantum ab initio calculations were performed to obtain the structures and action spectra of the complexes, which are directly compared with the experimental results.