Real time spectroscopic ellipsometry (RTSE) was used to monitor and control the depth of etching into a bulk GaAs wafer. Lateral interference due to patterning is the mechanism by which this optical technique, normally used for thin film measurement, can determine etch depth into bulk material. Scalar analysis permits fast data fitting and real time control. GaAs wafers were patterned with photoresist in line or square patterns with periods of 10, 20, or 40 mum, and etched in a solution of citric acid-hydrogen peroxide-de-ionized water. RTSE data were collected and simultaneously analyzed for etch depth. When the depth reached a preselected target value of up to 1.6 mum the etch was stopped. Final etch depth as measured by scanning electron microscopy was always within 5% of the target depth. Ex situ spectroscopic ellipsometry analysis of the etched GaAs, with the photoresist removed, also agreed well with the RTSE results.