The ultraviolet absorption spectrum of acetylperoxy radical was remeasured in the 195-280 nm range. Acetylperoxy radicals were generated by laser flash photolysis of Cl-2/CH3CHO/O-2 mixtures. The absorption cross sections were calibrated against the ethylperoxy radical, generated in the flash photolysis of Cl-2/C2H6/ O-2 mixtures. The acetylperoxy spectrum is bimodal, with a strong maximum at 207 nm, sigma = 6.67 x 10(-18) cm(2) molecule(-1), and a weaker maximum near 240 nm, sigma = 3.21 x 10(-18) cm(2) molecule(-1). Newly obtained cross sections were used along with absorption-time profiles, obtained over a range of radical concentrations, to determine a rate constant for the acetylperoxy self reaction at 298 K, 2CH(3)C(O)O-2 --> 2CH(3)C(O)O + O-2, of k(1) = (1.36 +/- 0.19) x 10(-11) cm(3) molecule(-1) s(-1), and to determine the rate coefficients for the acetyl- and methylperoxy cross reactions at 298 K, CH3C(O)O-2 + CH3O2 --> CH3C(O)O + CH3O + O-2 and CH3C(O)O-2 + CH3O2 --> CH3C(O)OH + HCHO + O-2, of k(4a) = (8.8 +/- 1.5) x 10(-12) cm(3) molecule(-1) s(-1) and k(4b) = (1.0 +/- 0.5) x 10(-12) cm(3) molecule(-1) s(-1), respectively. New cross sections are reported for CH3O2 and HO2 and the absorption spectrum of CH3COCl is presented.