We have developed a stable (< 1% drift/h at 1 angstrom/s), fast (approximately 200 ms), sensitive (S/N approximately 10-200 at 1 angstrom/s) evaporation rate monitor for controlling electron beam sources. Based on dual beam atomic absorption spectroscopy (AAS), in which a reference arm compensates for drift in the light source and signal detection apparatus, this technique is very wavelength and hence element specific, allowing many elements to be simultaneously and independently monitored. Furthermore, the system can operate at very high background gas pressures, as well as under ultrahigh vacuum conditions. Also, because only the light must enter the vacuum chamber and pass through the evaporant, minimal periodic maintenance inside the chamber is necessary. The versatility and sensitivity of this AA system make it a viable candidate for in situ monitoring of various other thin film processes, including sputtering, ion milling, and reactive ion etching.