Exptl. and theor. studies on the ultrafast photoinduced decompn. of three tert-Bu peroxides of general structure R-C(O)O-O-tert-Bu with R = phenyloxy, benzyl, or naphthyloxy in soln. are presented. Photoinduced O-O bond scission occurs within the time resoln. (200 fs) of the pump-probe expt. The subsequent dissocn. of photochem. excited carbonyloxy radicals, R-CO2, was monitored on a picosecond time scale by transient absorption at wavelengths between 290 and 1000 nm. The measured decay of R-CO2 is simulated via statistical unimol. rate theory using mol. energies, geometries, and vibrational frequencies obtained from d. functional theory (DFT) calcns. The results are compared with recent data for tert-Bu peroxybenzoate (R = phenyl). While benzoyloxy radicals exhibit nanosecond to microsecond lifetimes at ambient temp., insertion of an O atom or a methylene group between the Ph or naphthyl chromophore and the CO2 moiety significantly decreases the stability and thus lowers the lifetime of the carbonyloxy radicals in soln. to picoseconds. The reasons behind this structural effect on decompn. rate are discussed in terms of barrier heights for decarboxylation on the ground-state potential energy surface and of a fast reaction channel via electronically excited states of carbonyloxy radicals. Arrhenius parameters are reported for thermal rate consts., k(T), of R-CO2 decarboxylation as deduced from modeling of the time-resolved exptl. data in conjunction with the DFT calcns.