The photochemistry and radical chemistry of two widely used photoinitiators have been studied in liquid and supercritical carbon dioxide solutions and compared to conventional solvents such as acetonitrile and toluene. A specially modified time-resolved electron paramagnetic resonance (TREPR) apparatus was used to detect free radicals produced from the triplet states of dimethoxyphenylacetophenone (DMPA, 1) and trimethylbenzoyldiphenylphosphine oxide (TMDPO, 2) after excitation of the parent compounds using a 308 nm excimer laser. The photochemistry and photophysics of these molecules in CO2 are similar to those observed in conventional solvents. In the presence of methyl methacrylate (MMA) monomer, both benzoyl (1a) and dimethoxybenzyl (1b) radicals from DMPA react to form adduct radicals 1c and 1d, which are indistinguishable by TREPR. The MMA concentration dependence and time dependence of these spectra are simulated and discussed. From 2, the trimethylbenzoyl radical 2a is much less reactive than the diphenylphosphinyl oxide radical 2b. Measurement of the TREPR line width at a constant delay time as a function of MMA concentration gives the rate of addition, k(add), of 2b to MMA ((5.5 +/- 0.5) x 10(7) M-1 s(-1) in liquid CO2; (6.1 +/- 0.6) x 10(7) M-1 s(-1) in supercritical CO2). These values are close to those observed in acetonitrile ((8.1 +/- 1.6) x 10(7) M-1 s(-1)) and toluene ((4.0 +/- 0.4) x 10(7) M-1 s(-1)).