Elementary reactions controlling the oxidn. of partially oxygenated hydrocarbons are important to modeling of their atm. chem. The branching ratio (r) of CH3O- (I) and CH2CHO- (II) in reactions with OH, of (1) OH- + CH3OH -> I + H2O and (2) OH- + C2H4O -> II + H2O were detd. using time-resolved laser-induced fluorescence (LIF). Whereas reaction 1 is a minor channel of the total reaction at 298 K (r1 = 0.11), the yield of reaction 2 is pressure-dependent and predicted to be dominant under tropospheric conditions. The subsequent reactions of I with NO (reaction 3) and of II with O2 (reaction 4) are also pressure-dependent, indicating a dominant recombination process. At pressures >250 mbar and at 298 K, the rate coeffs. of these reactions are k3 = 2.5 * 10-11 and k4 = 2.5 * 10-13 cm/mol/s.