Electronically excited states of Mg + (NH 3 ) are studied by photodissociation after mass selection. The dissociation spectrum shows relatively sharp vibronic transitions centered at about 28000 and 36000cm - 1 . These absorption bands are assigned to the 2 P- 2 S type transitions localized on the Mg + ion. In photodissociation, a photoinduced charge-transfer process to produce NH 3 + is observed in addition to evaporation and intracluster reaction processes to produce Mg + and MgNH 2 + ions, respectively. The mechanism for the production of these ions is discussed in terms of the predissociative and non-adiabatic interactions between the low-lying states.