The formation of positively charged mixed metal oxide ions produced by alkali and alkaline earth chromate laser ablation has been investigated by Fourier transform ion cyclotron resonance mass spectrometry. The development of the fingerprints of these compounds with the increase of the counter ion radius, modification of power density, wavelength, or hydration degree allow us to gain a better understanding of the processes involved in ion formation and occurring after laser irradiation. The mixed metal ions are in all probability formed by ion/molecule reactions corresponding to sequential addition of neutral molecules on precursor ions. The ternary system of chromate compounds (chromium-oxygen-chromate counter ion) induced the presence of various neutral species in the gaseous cloud above the analyte surface; these species are responsible for the occurrence of competitive aggregation reactions. In this case, the formation of a given ion is governed by the relative affinity of its precursor ion with either neutral species. Moreover, the evolution of the alkali chromate fingerprint with the power density pushes us into considering the aggregation processes as equilibrium, the reverse way depending on the accumulated internal ion energy. By considering a simple model based on the minimization of unpaired electrons in favor of a greater stability, we additionally put forward two ionic structures for the most intense cluster ion M 3 CrO 4 ϩ observed on the fingerprint of alkali chromate. (Int J Mass Spectrom 198 (2000) 213-234) © 2000 Elsevier Science B.V.