We have produced cation-molecule complexes Mg+-(FCH3)(n) (n = 0-6) in a laser-ablation pickup source. A photo-induced reaction in Mg+-FCH3 was observed for the first time, with an exclusive production of CH3+ and MgF. This reaction selectivity is remarkable considering the fact that other product channels (CH3 + MgF+) and (Mg+ + FCH3) are much more energetically favorable. We have measured the relative photodissociation product yield as a function of the excitation wavelength in a broad spectral region. The reaction mechanism is considered to involve a curve crossing to an ionic PES, leading to the formation of the final products CH3+ and MgF.