The lateral etch rate of the epitaxial lift-off (ELO) process was determined as a function of the total HF concentration Cup and the O 2 partial pressure Po 2 . For this purpose samples were grown by metallorganic chemical vapor deposition and etched using a weight-induced ELO process. It was found that the etch rate increases linearly with C HF , which is in accordance with the model on the ELO process presented in a previous paper. This result and composition calculations of HF solutions show that the first step in the etch process of AlAs with an HF solution most probably takes place by chemical attack of undissociated HF on AlAs surface bonds. Furthermore, it is shown that the ELO rate increases slightly over a Po 2 range varying from 0.046 to 0.98 atm and that for Po 2 = 0.003 atm, a significantly lower etch rate is found. We suggest that the observed decrease is the result of surface passivation by elemental arsenic, which is formed by the reaction of AlAs with H + . An oxygen-poor atmosphere may allow the build-up of elemental arsenic on the surface, thus slowing down the AlAs reaction with HF. Oxygen, by removing arsenic as As 2 O 3 , keeps the surface active.