Wet etches find common application for accessing specific layers within a heterostructure. We investigate wet etching mechanisms during a phosphoric acid etch used to fabricate optoelectronic devices in AlGaAs heterostructure. Using a pulsed-laser in a reflectometer eliminates photoinduced etching and, as a consequence, accurately monitors the condition of the etchant in addition to measuring the etch rate, etch depth, and surface roughness. The study reveals an anomalous reflectometer signal for aged solutions related to the dynamics of the etch mechanism in establishing a transformation (reaction) layer at the exposed surface of the heterostructure. (C) 2003 The Electrochemical Society.