The Deacon process is composed of a number of discrete steps whose thermochemistry (Delta H, Delta S) is examined for a large number of main groups and transition metal oxides which may show catalytic behavior. The overall process comprises a catalytic cycle which can be examined in terms of two independent steps : (1) HCl absorption by a metal oxide to form metal chloride (or oxychloride) plus water and (2) oxidation of chloride by O-2 to regenerate metal oxide and free Cl-2. It is shown that in the temperature range 150-450 degrees C the exothermic chlorination step (1), if too exothermic, will be followed by a very slow endothermic oxidation step since Delta H-1 + Delta H-2 = -13.6 kcal, the overall enthalpy change in the process. An examination of the thermochemistry of a large number of main group and transition metals shows that only the latter have values of Delta H-1 and Delta H-2 that are sufficiently small as not to render the entire process too slow. Some experimental observations are reported on the behavior of six transition metal oxides and two main group oxides in steps 1 and 2. Step 1, the chlorination, is fast and exothermic for all metal oxides. Step 2, the oxidation, is very slow for all the metal chlorides except for Cu, below 450 degrees C. For the two main group metals examined, Al2O3 and MgO, no evolution of chlorine from the chlorides is observed below 750 degrees C in the presence of 1 atm of O-2(g).