Palladium(0) complexes facilitate many catalytic transformations that begin with the oxidative addition of a halobenzene. The ligation state of the palladium during this reaction is a vexing issue, owing to the inherent difficulty of isolating reactive, coordinatively unsaturated metal complexes. By isolating them in the gas phase in an ion-trap mass spectrometer, the reactivity of mono- and bisligated palladium complexes can be directly compared, and the former proved to be several orders of magnitude more reactive towards halobenzenes. Calculations of barrier heights for the oxidative addition led to additional experiments, which demonstrated that although the reaction proceeded to completion for iodobenzene, the reaction was slower for bromobenzene and progressed only as far as an ion–molecule adduct for chloro- and fluorobenzene.