The holographic photoelectrochemical etching of n-InP (100) samples in aqueous 1 M HCl was monitored optically using the synchronous detection of two wave mixing from the interfering beams. This signal was employed to operate a negative feedback system which stabilizes the projected interference pattern. The use of the stabilization system permits the recording of high spatial frequency gratings (<0.5 mu m), though their depth is limited to the value at which the self-diffracted signal passes through zero, It was shown that in the range where scalar theory may be applied, optical monitoring can be used to determine the absolute depth and the rate of relief development. Using this method the influence of sample orientation on the etching rate was studied, and some aspects of the etching process are discussed.