A series of rare-earth-(RE-)doped α-sialons (RExSi12–4.5xAl4.5xO1.5xN16–1.5x, with x = 0.40 for RE = Nd, Sm, Yb, and x = 0.48 for RE = Y) were prepared and heat-treated in air at 1350 °C for 66–727 h (3–30 days), and the variations in composition and structure with time of the formed oxide scales and matrix materials were investigated. In the oxide scales of the Nd-, Sm-, and Y-containing samples a liquid was formed, apparently in (quasi-)equilibrium with the crystalline phases cristobalite and mullite, while only crystalline Yb2Si2O7, cristobalite, and mullite were observed in the Yb sample. Apparently, the liquid plays an important role in the oxidation process. In the depleted zone, located between the scale and the matrix, the liquid attacks the matrix phases, and a process takes place in which the originally formed phases dissolve and reprecipitate as more oxygen-rich phases. In the Nd- and Sm-doped systems, where the α-sialon phase is inherently metastable at 1350 °C, an extensive α → β-sialon transformation takes place, creating still more liquid. As a consequence, the oxidation resistance of α-sialons containing Nd and Sm is much lower than those containing Y and, in particular, Yb.