Absolute rates of CH2(a1A1) removal by the olefins ethene (C2H4), propene (C3H6), isobutene (i-C4H8), butadiene (C4H6), allene (C3H4) and the aromatic hydrocarbons toluene (C6H5CH3), p-xylene (C6H4(CH3)2), ethylbenzene (C6H5C2H5) and cumene (C6H5–(i-C3H7)) have been determined at low pressures and room temperature. CH2(a1A1) was produced by pulsed laser photolysis of CH2CO. Concentration profiles of CH2(a1A1) were recorded using laser induced fluorescence with a variable time delay between the probe laser and the photolysis laser. —The contributions of physical deactivation to the removal of CH2(a1A1) in the reactions with C2H4, C3H6, C3H4 and C6H5CH3 were investigated quantitatively via the formation of CH2(X3B1), detected by LMR. —1CH2 is removed very efficiently by all the reactants, with second order rate constants in units of 1014 cm3/mol sk(C2H4) = 1.4, k(C3H6) = 2.0, k(i-C4H8) = 2.0, k(C4H6) = 2.1, k(C3H4) = 1.9 and k(C6H5CH3) = 2.3, k(C6H4(CH3)2) = 2.2, k(C6H5C2H5) = 2.5, k(C6H5(i-C3H7)) = 2.6. The branching ratios of physical deactivation versus total removal for C2H4, C3H6, C3H4, C6H5CH3 were 0.20, 0.24, 0.30, 0.29. The dependence of reaction rates on alkylic substitution of the olefinic and aromatic II bonding systems is small.