Picosecond UV pump (248.5 nm)/IR probe spectroscopy has been applied to the study of the decomposition of several aromatic diacyl peroxides, peroxycarbonates, and of a peroxyester dissolved in CH2Cl2. Measuring the IR transient absorbance in the 2100 to 2450 cm-1 range allows to monitor the formation and vibrational relaxation of the photoproduct CO2 via the asymmetric stretching mode (n3) with a time resolution of 1.8 ps.With each of the six peroxides a CO2 molecule is released at delay times below 10 ps. The energy relaxation of the initially formed vibrationally hot CO2 is followed over the time range up to 500 ps. Analysis of the transient IR spectra, via an anharmonic oscillator model proposed by Hamm et al. [1], shows a monoexponential decay of internal energy. Irrespective of the type of peroxide a single relaxation time of 67 ± 5 ps is found to adequately represent the cooling behavior of CO2 in liquid CH2Cl2. The “initial” temperatures of vibrationally hot CO2 at a delay time of 20 ps after applying the pump pulse differ considerably, between 1400 and 2700 K for the decompositions of tert-butyl benzoyl peroxide and tert-butyl benzoyl carbonate, respectively. These remarkably high temperatures are assumed to originate from energy release associated with structural relaxation of the bent OCO moiety to form the linear CO2 molecule.