The reactions of CH3C(O)O-2 with itself and with CH3O2 are investigated using flash photolysis combined with time-resolved UV spectroscopy and transient infrared absorption. The UV spectrum of CH3C(O)O-2 exhibits two bands; the stronger short wavelength component has a maximum cross section of 6.5 x 10(-18) cm(2) at 206 nm and the weaker one a cross section of 2.9 x 10(-18) cm(2) at 250 nm. These bands are used to monitor the disappearance of CH3C(O)O-2 and the secondary formation of CH3O2, yielding a self-reaction rate constant (3.0(-1.1)(+1.7)) x 10(-12) e((504+/-114/T) cm(3) s(-1) and a cross reaction rate constant of (8.5(-2.0)(+2.6)) x 10(-13) e((726+/-75)/T) cm(3) s(-1). Transient IR monitoring of the formation of CH2O reveals CH3C(O)O-2 + CH3O2 --> CH2O + CH3COOH + O-2 to be the dominant channel for the cross reaction over the entire 209-358 K temperature range. This contrasts with previous studies that indicate a steep temperature dependence for the branching ratio, with a channel leading to CH3C(O)O + CH3O + O-2 dominant at high temperatures.