Certain aspects of the electrochemical deposition of Al on Au electrodes from ambient-temperature acidic AlCl3/1-ethyl-3-methyl imidazolium chloride (EMIC) melts have been examined in situ using a quartz crystal microbalance (QCM). These experiments were performed using benzene as a cosolvent to decrease the very high viscosity of the neat molten salt. A linear correlation was found between the change in the frequency of the QCM and the amount of electrodeposited Al for amounts of Al ranging from 0.05 to 0.27 mug/cm(2). This observation suggests that within this mass range the films are relatively smooth and rigidly bound to the underlying substrate. For lower masses, the plot was also linear, except that the slope was significantly smaller than that expected based on the weight of Al, a phenomenon attributed to mass compensation. Large deviations from linearity were observed when the amount of deposited Al increased beyond 0.27 mug/cm(2). This behavior is consistent with the growth of discrete Al clusters leading to increased surface roughness and to liquid entrapment within the cluster array, which contributes to the frequency changes measured by the QCM. Long times were required for the mass of the Au electrode to return to its original value following deposition and stripping, which is consistent with the formation of bulk Al/Au alloys as has been earlier proposed.