In the present work a so-called diffusion and reaction related model (DR model) is derived based on the notion that the overall etch rate in the epitaxial lift-off (ELO) process is determined both by the diffusion rate of hydrofluoric acid to the etch front and its subsequent reaction rate in the process. In contrast to the model that was previously described in the literature, the DR model yields etch rates which are in quantitative agreement with those obtained experimentally. In order to verify the DR model, the ELO etch rate of AlAs1-yPy release layers is determined as a function of the phosphorus percentage, the release layer thickness and the temperature. In accordance with the DR model, it is shown that the etch rate is reaction rate related by the dependence on the phosphorus percentage in the release layer, and that the etch rate is diffusion rate related by the dependence on the release layer thickness. From the temperature dependence, an activation energy of 0.31 eV could be deduced for the ELO process under the present conditions.