The effect of analyte surface concentration on the observed extent of beam-induced dehalogenation in liquid secondary ion mass spectrometry (LSIMS) was investigated using a three-pronged approach. The thrust of the experiments was to vary directly and indirectly the analyte surface concentration and monitor the extent of dehalogenation. These experimental approaches consisted of the use of an anionic surfactant as a doping agent, synthetically modifying the hydrophobicity of a model analyte, and changing matrix surface tension through suitable matrix liquid selection. The results obtained show that increased analyte surface concentration leads to an increased extent of dehalogenation. These results are in agreement with the postulate that beam-induced reactions occur mainly at or near the surface, where the concentration of reactive species generated by the interaction of the bombarding particles and the sample is highest. It is proposed that this relationship is applicable to other phenomena in fast atom bombardment (FAB)/LSIMS. Copyright © 1998 John Wiley & Sons, Ltd.