Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is an efficient, sensitive method for characterizing semiconductor surfaces. In addition, TOF-SIMS can be applied in a depth profiling mode allowing qualitative characterization of the top 20 nm of material. The utility of TOF-SIMS ultra-shallow depth profiling is demonstrated on GaAs substrates that were passivated with P2S5 solutions and oxidized by exposure to an UV/ozone treatment.