Traces of particulate mercury sulfide in stabilized crude oils transform to elemental mercury below 400°C in refinery distillation units. We evaluated this transformation and measured reaction rates and activation energies in crude oil and mercury spiked mineral oil using glass vessel at atmospheric pressure and a microunit at 1000 psig. Results show that mercury sulfide decomposes irreversibly above ˜150°C. The data indicates the presence of two mercury species. Approximately 88% of the mercury sulfide decomposed with an activation energy of 56±7 kJ mol ^-1. The remaining 12% decomposed with an activation energy of 130±10 kJ mol ^-1. Near-identical kinetic parameters were found with differential measurements at atmospheric pressure and with integral measurements in the liquid phase at over 1000 psig, indicating that mild pressures do not change the reaction kinetics. The results show that >99% of mercury sulfides in crude oil decompose in a typical crude distillation unit.