We report on the oxidative stability of thiol-passivated Au monolayer-protected clusters (MPCs) made via a modified Brust-Schiffrin method. A sequential oxidation of the anchored thiol groups to disulfide and sulfonate groups and the oxidation of Au atoms to Au3+ species is observed upon exposure of Au MPCs to air in the presence of halide anions. In addition, the average nanoparticle size grows via aggregation of the MPCs, leading eventually to partial oxidation of the Au MPCs and precipitation of the remaining nanoparticles from solution or to complete oxidation of the gold atoms at high halide concentrations. These results show that Au MPCs are prone to oxidation in air in the presence of halide anions, and therefore, particles made using phase transfer reagents such as tetraoctylammonium bromide must be thoroughly removed to avoid particle size growth, oxidation, and precipitation of the Au MPCs. In addition, for biological applications involving Au MPCs, care must be taken to ensure that oxidation of MPCs in air is not problematic when working in media containing halide anions.