This study documents the influence of different surface conditions produced by various heat treatments on the in vitro degradation performance of an Mg-Y-RE alloy (WE43) investigated by immersion in simulated body fluid. WE43 samples were, respectively (i) annealed at 525 degrees C (plus artificial aging at 250 degrees C in one case) and afterwards polished; and (ii) polished, annealed at 500 degrees C in air and subsequently investigated in the oxidized state. Thermogravimetric analysis (TGA) indicates a mass gain during oxidation in air, following a square-root law over time. X-ray diffraction spectra imply a growing Y(2)O(3) layer upon oxidation, and Auger electron spectroscopy depth profiles show an increased oxide layer thickness which develops according to the behavior observed by TGA. Macroscopically, the degradation performance of the differently heat-treated samples can be divided into two groups. Annealed and polished samples show a fast and homogeneous degradation which slows with time. Their degradation behavior is approximated by a parabolic law. Oxidized samples exhibit a slow initial degradation rate which increases when the protection of the oxide layer is reduced. Overall, they reveal a sigmoidal degradation behavior. Here the differing degradation performances of the annealed-polished and the oxidized samples are related to the different surface conditions and explained on the basis of a depletion hypothesis.