The differential heat of adsorption of oxygen on activated cocoanut charcoal has been measured over the low concentration range to 14 micromoles per gram, by means of an improved ice calorimeter and a new charcoal bulb, in which oxygen is introduced into the middle of the charcoal mass. At zero concentration, a heat of adsorption of 129.0 kcal. per mole of oxygen was found, dropping to 72.0 kcal. at approximately 6.0 micromoles per gram. The initial heat of adsorption is much higher than the highest value that has been obtained previously. This indicates that the plain charcoal bulb previously used permitted loss of heat from the calorimeter; consequently, the values for the initial heats of adsorption were low. The theoretical significance of the high initial heat is discussed briefly.