A novel method for simultaneous determination of the decomposition rate constant, k(D) and the initiator efficiency, f, of a free-radical polymerisation initiator has been developed. The decomposition is carried out in a continuous-stirred-tank reactor (CSTR) in the presence of a radical scavenger. Mass balances for the initiator and the scavenger show that both f and k(D) can be determined from a set of constant-temperature experiments with a varying reactor mean residence time. Experiments were conducted in supercritical carbon dioxide (scCO(2)) using diethyl peroxydicarbonate (DEPDC) as the free-radical initiator and galvinoxyl as the free-radical scavenger. The decomposition of DEPDC was first-order between 338 and 358 K. The activation energy, E-A was 132 kJ/mol, the pre-exponential factor, A(D), was 6.3 x 10(16) s(-1), and the initiator efficiency,f, was 0.6. These values are similar to those reported in the literature for DEPDC decomposition in traditional organic solvents. Varying the carbon dioxide pressure between 240 and 310 bar at 348 K had no effect on the Value of k(D). Transition state theory predicted no significant effect of pressure or solvent composition on the initiator decomposition rate constant.