The adsorption of a charged thiol (HSC10COO-) on gold from its aqueous solutions of different concentration was monitored in situ by probing the surface charge. This was accomplished by measuring the interfacial forces between a modified (with a negatively charged silica sphere) tip of an atomic force microscope and the thiol-adsorbed gold surface as a function of adsorption time. The surface charge and potential were then deduced from the force data. If a Langmuir rate law is employed to fit the overall surface coverage vs time data, average observed adsorption rate constants of 0.045 +/- 0.005 (0.5 mM) and 0.020 +/- 0.003 (0.05 mM) min(-1) were obtained and were dependent on thiol concentration. The self-assembly process was a two-step process, an initial fast step followed by a slow step.