The heat capacity at constant pressure (C-p) of mullite (single crystalline and dense and powdered polycrystalline) was determined between -125degrees and 1400degreesC. Measurements were performed in three laboratories using two different disk-type differential scanning calorimeters (DLR and Netzsch) and one cylinder-type (Setaram). On the basis of these experimental data a new master curve C-p(T) is presented. Up to about 1100degreesC the C-p curve agrees well with those from previously published results: The C-p master curve displays a parabolic shape with C-p values of about 0.415 J/(g.K) at -125degreesC, 0.78 J/(g.K) at 25degreesC, and 1.25 J/(g.K) at 1000degreesC. At temperatures above 1100degreesC a yet unknown, steplike, weak C-p increase is observed with DeltaCdegrees step values ranging between 0.03 and 0.10 J/(g.K). This C-p anomaly of mullite is reproducible under various heating conditions and is reversible both on heating and on cooling. The extrapolated onset temperature of the anomaly (T-on.) is controlled by the crystalline state of the samples (single crystal, T-on. above 1100degreesC; polycrystal, T-on. above 1250degreesC) but does not depend on the composition of mullite. Sillimanite, which is structurally closely related to mullite, displays no C-p anomaly. The absence of latent heat (DeltaH = 0) and the shape of the C-p anomaly of mullite are comparable to the behavior of glass-forming materials at the glass transition. The observations can best be interpreted in terms of an onset of ion-jumping between adjacent structural sites T and T* and O(C) and O(C)*. Other interpretations, i.e., clockwise and counterclockwise rotations of structural units, or minor decomposition of mullite to alpha-Al2O3 plus SiO2 glass observed at the surface of the single crystals, are less probable.