The thermal decomposition of the organic free-radical initiator, diethylperoxydicarbonate (DEPDC), was monitored by in situ ATR-FT-IR in heptane, and in the green solvent supercritical carbon dioxide (scCO(2)) both with and without supercritical ethylene. It was observed that the characteristic peaks of DEPDC at 1802-1803 and 1194-1203 cm(-1) decreased significantly upon heating corresponding to the decomposition of DEPDC, while two new intense peaks Simultaneously appeared at 1747 and 1262 cm(-1) in heptane, and similarly at 1756 and 1250 cm(-1) in ScCO2. The changes in the absorbance intensity of the characteristic peaks of the initiator during the decomposition were used for the measurement of the decomposition rate constant (k(d)) of DEPDC. It was found that the thermal decomposition of DEPDC at low concentration in either heptane under atmospheric N-2 or scCO(2) under high pressure was via the first-order kinetics of unimolecular decomposition. The activation energy of the thermal decomposition of DEPDC was found to be 115 kJ/mol in heptane from 40 to 74 degrees C and 118 kJ/mol in ScCO2 from 40 to 60 degrees C. These new peaks revealed the formation of carboxyl groups contained in the decomposed products, indicating incomplete decarboxylation. During removal of CO2 after the reaction in scCO(2), the instable intermediate monoethyl carbonate was decarboxyiated and converted into the major end product, ethanol. (c) 2006 Elsevier Ltd. All rights reserved.