The thermodynamic properties of magnesium hydride nanoparticles have been investigated by hydrogen decomposition pressure measurements using the Sieverts technique. A mechanochemical method was used to synthesize MgH2 nanoparticles (down to similar to 7 nm in size) embedded in a LiCl salt matrix. In comparison to bulk MgH2, the mechanochemically produced MgH2 with the smallest particle size showed a small but measurable decrease in the decomposition reaction enthalpy (Delta H decrease of 2.84 kJ/mol H-2 from Delta H-bulk = 74.06 +/- 0.42 kJ/mol H-2 to Delta H-nano = 71.22 +/- 0.49 kJ/mol H-2). The reduction in Delta H matches theoretical predictions and was also coupled with a similar reduction in reaction entropy (AS decrease of 3.8 J/mol H-2/K from Delta S-bulk = 133.4 +/- 0.7 J/mol H-2/K to Delta S-nano = 129.6 +/- 0.8 J/mol H-2/K). The thermodynamic changes in the MgH2 nanoparticle system correspond to a drop in the 1 bar hydrogen equilibrium temperature (T-1 bar) by similar to 6 degrees C to 276.2 +/- 2.4 degrees C in contrast to the bulk MgH2 system at 281.8 +/- 2.2 degrees C. The reduction in the desorption temperature is less than that expected from theoretical studies due to the decrease in Delta S that acts to partially counteract the effect from the change in Delta H.