Mixed clusters were produced by expansion of an NO/CH4/Ar/H2O mixture (5%, 5%, 90%, trace) in a supersonic jet. Ionization of the clusters with 30 ps, 266 nm laser pulses resulted in the virtually 100% conversion to NO+ (N2O3)(n) and NO2+(N2O3)(n) cluster ions subsequently detected in a time-of-flight mass spectrometer. Some incomplete conversion was evidenced by the detection of (NO)(m)(+)(N2O3)(n) ions with m > 1. The identity of the product ions was confirmed by the use of isotopically substituted nitric oxide and the role of the water was investigated using O-18-substituted H2O. Possible photochemical mechanisms are discussed.