The quartz crystal microbalance (QCM) is mainly used to monitor deposition from aqueous media and has been used only rarely to monitor deposition from organic media. We report the use of a QCM to monitor the formation of a self-assembled monolayer of 11-mercapto-1-undecanol on the surface of gold. This process was selected because it has been well studied by ex situ methods, such as ellipsometry and contact angle measurement. Because the layer deposited is thin and behaves fairly elastically, the change in frequency could be converted validly to mass deposited on the gold surface, thereby providing an average value for packing density. The value of 3.74 � 0.47 chains/nm² determined in our laboratory compares reasonably well with the few existing experimental values determined from diffraction data and from electrochemical data. In addition, the QCM studies verified the two-stage kinetics found by those using ex situ methods: fast deposition of most of the mass followed by a very slow consolidation to form a stable monolayer.