A molecular tracer method for scanning tunneling microscopy allowed the observation of individual molecular events at a crystal surface in solution. Long-chain alkanes or ethers were coadsorbed with similar length thioethers on a graphite surface. Due to the distinctive contrast of the sulfur atoms, it was possible to track individual thioether tracer molecules. The exchange of tracer molecules between the adsorbed monolayer and the overlying solution was measured at equilibrium. Rates depended on chain length, temperature, and neighboring molecules. Tracers at boundaries between crystal domains revealed specific molecular processes driving boundary fluctuations and domain growth. In addition to adsorption-desorption driven rearrangements, collective motion within the monolayer has been seen. Longer chain ethers exhibit two phases having different molecular configurations at grain boundaries and different microstructural. habits. Transition between phases was promoted by the scanning probe.