Yellow-emitting alpha-sialon:Eu2+ phosphors have been reported as interesting down-conversion luminescent materials in white LEDs. In this work, the thermal quenching of alpha-sialon:Eu2+ with the compositions of Mval+(m/val+)Si12-(m+n)Alm+nOnN16-n (M=Ca, Mg, Lu) is studied by investigating the effects of chemical composition, activator concentration, and substitution cation on the temperature-dependent luminescence. The chemical composition of alpha-sialon:Eu2+ was varied in a wide range (0.5 < m < 2.0, 1 < n < 1.8). It shows that the m value significantly affects the thermal quenching of alpha-sialon, whereas the n value hardly does. This difference is ascribed to the obvious lattice expansion and the increase of absolute activator concentration as m increases. The thermal quenching increases with increasing the Eu2+ concentration, which is due to enhanced Stokes shift. The type of substitution cation also has an influence on thermal quenching. Among the substitution cations in this work, Lu-alpha-sialon:Eu2+ exhibits largest thermal quenching. Photoionization is considered as the mechanism for the thermal quenching of Lu-alpha-sialon: Eu2+.