The adsorption of I- and Br- have been examined on polycrystalline gold in 50 mM NaClO4. The mass of the adsorbed layer was measured in-situ with a quartz crystal microbalance using one of the gold electrodes on the oscillator as the working electrode in an electrochemical cell. In the concentration range studied, 0.1 mM to 10 mM, both iodide and bromide were found to adsorb to full monolayer coverage. The electrosorption valencies γ, were calculated from plots of charge versus coverage and were found to be 1.01 ±0.05 and 0.39 ±0.03 for iodide and bromide respectively. © 1988.