During epitaxial lift-off of II-VI semiconductors a sacrificial layer of MgS is dissolved by acid. Here we show that the etching speed of this process varies inversely as the square root of the layer thickness, following a model developed previously for III-V lift-off where the rate limiting step in both cases is transport of insoluble product gases from the etching layer. We also propose a model to explain why sacrificial layer etching fails when strong cohesive forces resist the lifting of the epilayer. This occurs when the sacrificial layer is too thin or when it contains more than a critical amount of an insoluble component, cohesion arising from dispersion forces or chains of insoluble atoms, respectively. Our method follows the original procedure [6] for (Ga,Al)As structures with an etchable AlAs layer but it was not clear if the same lift-off mechanism operates in both cases. In section 2 we demonstrate that this is the case. In section 3, we suggest a new model to explain why the lift-off technique fails either if the etchable layer thickness, t is below a minimum value, or contains an insoluble component (either GaAs or Zn(S,Se)) above a critical concentration.