Many models of foulant deposition on a membrane surface during reverse osmosis (RO) are based on flux-decline data and scanning electron microscopy (SEM) to predict the mechanism of fouling. The actual mechanism of calcium sulfate deposition remains incompletely understood due to the lack of additional techniques to detect the calcium sulfate deposition during RO. An in situ ultrasonic testing technique has been employed extensively to detect calcium sulfate deposition and growth on a RO membrane surface. Crossflow and dead-end operations were conducted with 2 g/l calcium sulfate solution. The experimental results show good correspondence between the ultrasonic response signal and the development of a calcium sulfate foulant layer on a membrane surface. In particular, a fouling echo obtained in the time domain indicates the actual state of a fouling layer on a membrane surface. An increase in the relative amplitude of the fouling echo results from the build-up of the fouling layer. In addition, the movement of the fouling echo in the time-domain occurred due to an increase in the thickness of the fouling layer. The flux decline is relative to the increases in both the density and thickness of the deposit. The changes in the density of a fouling layer as well as the thickness can be substantiated by a predictive modeling program-ultrasonic reflection modeling. The predicted results were in good agreement with the actual observations obtained during ultrasonic testing of membranes.