The thermoelastic parameters of synthetic Mn3Al2Si3O12 spessartine garnet were examined in situ at high pressure up to 13 GPa and high temperature up to 1,100 K, by synchrotron radiation energy dispersive X-ray diffraction within a DIA-type multi-anvil press apparatus. The analysis of room temperature data yielded K (0) = 172 +/- A 4 GPa and K (0) (') = 5.0 +/- A 0.9 when V (0,300) is fixed to 1,564.96 (3). Fitting of P-V-T data by means of the high-temperature third-order Birch-Murnaghan EoS gives the thermoelastic parameters: K (0) = 171 +/- A 4 GPa, K (0) (') = 5.3 +/- A 0.8, (a,K (0,T) /a,T) (P) = -0.049 +/- A 0.007 GPa K-1, a (0) = 1.59 +/- A 0.33 x 10(-5) K-1 and b (0) = 2.91 +/- A 0.69 x 10(-8) K-2 (e.g., alpha (0,300) = 2.46 +/- A 0.54 x 10(-5) K-1). Comparison with thermoelastic properties of other garnet end-members indicated that the compression mechanism of spessartine might be the same as almandine and pyrope but differs from that of grossular. On the other hand, at high temperature, spessartine softens substantially faster than pyrope and grossular. Such softening, which is also reported for almandine, emphasize the importance of the cation in the dodecahedral site on the thermoelastic properties of aluminosilicate garnet.