In the in situ Gasification Chemical Looping Combustion of coal (iG-CLC), the fuel is gasified in situ in the fuel reactor and gasification products are converted to CO2 and H2O by reaction with the oxygen carrier. This work is the first study on mercury release in Chemical Looping Combustion of coal. The determination of mercury speciation (Hg-0/Hg2+) in fuel and air reactors as well as the quantification of the amount of mercury released in each reactor were addressed for the first time. The fraction of the mercury in coal vaporized in the fuel reactor depended mainly on the fuel reactor temperature and the coal type. In the fuel reactor, mercury was mainly emitted as Hg-0 in the gas phase and the amount increased with the temperature. In the air reactor, mercury was mostly emitted as He2+. In a real CLC system, mercury emissions to the atmosphere will decrease compared to conventional combustion as only mercury released in the air reactor will reach the atmosphere. However, measures should be taken to reduce Hg-0 in the CO2 stream before the purification and compression steps in order to avoid operational problems.