In this paper, luminescence thermal quenching of M2SiO4:Eu2+(M=Sr, Ba) orthosilicate phosphors and mechanisms for thermal quenching proposed by different authors are briefly reviewed. Depending on preparation conditions and/or Eu2+-doping concentrations, the quenching temperature (T1/2) and acti-vation energy for thermal quenching of the same orthosilicate phosphor reported by different authors are inconsistent. The common conclusion is that T1/2 of the intermediate compound (Ba1-xSrx)2SiO4:Eu2+(x≈0.5) is higher than that of either Sr2SiO4:Eu2+or Ba2SiO4:Eu2+end-member. Moreover, T1/2 of the best-performing SrBaSiO4:Eu2+ is evidently lower than that of YAG:Ce3+ and some Eu2+-doped nitride phosphors. Regarding the quenching mechanism, most of the investigators attributed thermal quenching to a thermally assisted 4f-5d cross-over in the configuration coordinate diagram. Only a few authors ascribed thermal quenching to a thermally assisted photoionization of 5d electron to conduction band of the host. Nonetheless, a close inspection of T1/2 and Stokes shift derived from the vibrational spectra of the intermediate compound and end-member phosphors indicates that the 5d electron photoionization model instead of the 4f-5d crossing decay model should be the genuine mechanism for the thermal quenching of M2SiO4:Eu2+(M=Sr, Ba) phosphors. Since the relationship between T1/2 and Stokes shift of the phosphors does not support the 4f-5d crossing decay model. The ionization probability of the 5d electron depends on the energy gap (EdC) between 5d1 level of the Eu2+and conduction band minimum (CBM) of the host at higher temperatures. Lattice thermal expansion would result in an elevating 5d1 level of the Eu2+along with a diminishing CBM of the host and as a consequence a reduction in EdC and an enhanced photoionization probability at elevated temperatures. A less rigid lattice and hence a larger coefficient of thermal expansion of M2SiO4 hosts should be the physical origin of poorer thermal quenching properties of the orthosilicate phosphors.