Chars were produced from two coals (North Dakota lignite and Pittsburgh No. 8 hva bituminous coal) in a flat flame burner at atmospheric pressure. Intrinsic char reactivities to oxygen were then measured in a high-pressure thermogravimetric analyzer as a function of temperature, total pressure, and partial pressure of oxygen. Care was taken to avoid mass transfer effects. Reactivities were normalized by the available mass of char at any given time (g/(g(avail).s)). Reactivity data were obtained over a wide range of char burnout and were found to be independent of burnout level between char burnouts of 20 and 60%. Data were obtained at pressures from 1 to 32 atm and at temperatures from 598 to 823 K and were analyzed using the nth order rate law and Arrhenius temperature dependence. The reaction order of each char was shown to be constant at 0.7 for total pressures ranging from 1 to 32 atm and for partial pressures of oxygen ranging from 0.03 to 12.8 atm, and was essentially the same for both chars. Activation energies determined for each char were also determined to be constant with total pressure and partial pressure of O-2.