The standard enthalpy change of the reaction AOH(g)→A(g)+OH(g) has been measured in H2+O2+N2 flames for all alkali metals A through a procedure based upon the third law. Values obtained for ΔH°° are 101±2, 77±4, 81±2, 83±2 and 91±3 kcal/mole for A = Li, Na, K, Rb and Cs respectively.