Spectroscopic ellipsometry (SE) is an accurate in situ method for determining the composition and thickness of III-V semiconductor layers during growth. In order to achieve this control, an optical constant database is used to compare the modeled and experimental ellipsometric data. This procedure is very effective for controlling thickness and composition when the film has been growing for some time but is usually unreliable until several minutes into the growth (corresponding to similar to 50 nm). To use SE for the control of thinner layers (1-20 nm), a different approach has to be used. A new strategy is proposed which consists in looking at the raw SE signal in a limited wavelength range where the signal varies almost linearly with the film thickness. For AlAs grown on GaAs, it is found the phase part of the SE signal for light between 2.5 and 2.8 eV varies nearly linearly with the AlAs thickness. A series of AlAs/GaAs multiquantum well structures are grown and analyzed. The thickness determined by this use of the ellipsometric data are in close agreement with independent thickness measurements obtained from high resolution x-ray diffraction.