We describe a simple technique for measuring the fill level of a liquid or solid precursor bubbler. This technique may be applied on chemical vapor deposition (CVD) systems which employ back-pressure control of the liquid/solid source. The method is based on the principle that the pressurization rate of the bubbler vessel is a measure of its unfilled volume; and this parameter can be easily and accurately measured using the electronic back-pressure controllers that are typically supplied on CVD reactors. As the precursor chemical is consumed, a larger gas volume is available within the bubbler, causing the pressurization rate to become slower. Over the life of the bubbler, the fill level can be inferred by comparing the bubbler's pressurization rate with that of the new source bubbler. The procedure can be used to estimate the source consumption within a few grams precision, exemplified here using a liquid trimethylgallium bubbler. While this procedure does not provide a continuous measurement of the bubbler fill, it works equally well for solid and liquid precursors; and the technique may be implemented into the reactor control system and employed to provide day-by-day measurement of the bubbler fill. (C) 2008 Elsevier B.V. All rights reserved.