The thermal stability of MgB2 has been studied experimentally to determine the role of thermodynamic and kinetic barriers in the decomposition process. The MgB2 decomposition rate approaches one monolayer per second at 650°C and has an activation energy of 2.0 eV. The evaporation coefficient is inferred to be ∼ 10-4, indicating that this process is kinetically limited. These values were inferred from in situ measurements using a quartz crystal microbalance and a residual gas analyzer, in conjunction with ex situ measurements of redeposited material by Rutherford backscattering spectroscopy and secondary ion mass spectroscopy. The presence of a large kinetic barrier to decomposition indicates that the synthesis of MgB2 thin films conditions may be possible with vacuum processing, albeit within a narrow window in the reactive growth conditions.