The metastable decomposition of photoionized NO/NH3 clusters has been investigated employing a reflectron time-of-flight mass spectrometer. The metastable dissociation spectra of the most prominent cluster ion species NO+(NH3)(n) (3 less than or equal to n less than or equal to 28) show that the rate coefficient for the loss of one NH3 molecule from the cluster rapidly increases up to n = 13 and thereafter is nearly independent of the cluster size. This is interpreted as evidence of a stable structure that consists of a central NO+ ion surrounded by a first solvation shell of 12 NH3 molecules.