A molecular tracer experiment has been performed to observe exchange of molecules between phases at the solid-liquid interface. Mixtures of long-chain molecules were adsorbed on a graphite surface from solution. The thioether hexadecyl sulfide (CH3(CH2)(15))(2)S was coadsorbed with sterically similar molecules hexadecyl ether (CH3(CH2)(15))(2)O or tritriacontane C33H68 These molecules cocrystallize but are readily distinguishable in STM images due to strong contrast between sulfur atoms and methylene or oxygen groups. Thioether molecules are seen to continually enter and leave the monolayer even in well-ordered crystalline regions. Molecules reside on the surface for periods of several seconds'. Despite continual replacement of molecules the images remain well-defined, indicating that each exchange is completed within a few milliseconds.