Aq. 35-60% H2O2 was added to CaO at 40 degrees C, (H2O2/CaO)(inlet) = 0.4-1.4 (by moles), to prep. mixts. contg. 5-16.5% active 0 in a dry (105-140 degrees C) CaO2. The ratio 0.8 gave the max. efficiency. The low H2O2 concns. and the amts. slightly less than stoichiometric resulted in a high deg., ca. 80%, of utilization of active O. The av. CaO2 product was CaO2 75.81 Ca(OH)(2) 24, active O 16.84, Ca 55.06%. At inlet ratios > 1 and H2O2 used in excess, active O exceeded 17%. Higher H2O2 concns. led to higher active O losses. The (H2O/CaO),(inlet) ratio was a factor more important than H2O2 concn.