Time-resolved ultraviolet (UV) spectroscopy is used to investigate the reaction between CH3C(O)O-2 and C2H5O2 at 130 Torr total pressure and over the 220-440-K temperature range. Deconvolution of the UV spectra yields concentration versus time profiles for the acetylperoxy radical and for the C2H5O2 + CH3O2 composite. The analysis of these data over short times, during which CH3C(O)O-2 radicals are present, yields a temperature-dependent rate constant of k(1) (5(-2)(+5)) x 10(-13)e((1070 +/- 200)/T) cm(3) s(-1) for the title reaction. The subsequent long-time decay of the ethylperoxy radicals that remain, after the reaction with CH3C(O)O-2 is complete, is faster than when acetaldehyde and, therefore, acetylperoxy radicals are not present. The secondary chemistry that potentially contributes to this decay is discussed.