The use of calcium peroxide (CaO2) powder as a source of H2O2 to promote modified Fenton (MF) chemistry was studied. First, the rate of production and yield of H2O2 from CaO2 dissolving in water at pH 6-9, and 12-13 (i.e., unbuffered CaO2) was measured. The rate of CaO2 dissolution increased as pH decreased, from 62 h for complete dissolution at pH 12-13 to only 4 h at pH 6. The yield of H2O2 also increased with decreasing pH, from zero at pH 12-13 to 82% at pH 6. The ability of CaO2 to promote MF oxidation of PCE was demonstrated with a hydroxyl radical ((OH)-O-center dot) scavenger (2-propanol) at pH 8. The scavenger inhibited PCE oxidation, but 97% of the PCE was oxidized without it. Release of Cl- showed that PCE was mineralized. Finally, PCE oxidation was compared with liquid H2O2 (pH 7) and with CaO2 (pH 6,7, 8, 9). Liquid H2O2 showed the lowest efficiency (mol H2O2 consumed/mol PCE oxidized) and the greatest temperature increase, disproportionation to O-2, and PCE volatilization. CaO2 was a more efficient oxidant than liquid H2O2 at all pH values because it only releases H2O2 upon dissolution, reducing the loss to O-2 and volatilization. CaO2 performed optimally at pH 8. (c) 2007 Elsevier B.V. All rights reserved.