The kinetics of the reaction of ground-state methylene radicals ((X) over tilde B-3(1), (CH2)-C-3) with methyl radicals (CH3) has been investigated. Both radicals were produced by the photolysis of acetone (CH3C(O)CH3) at 193 nm. Using time-resolved time-of-flight mass spectrometry, the temporal evolution of the concentration of reactants as well as products could be observed simultaneously. Rate coefficients at T = (300 +/- 3) K with a bath gas (He) pressure of P = 133 Pa (1 Torr) for (CH2)-C-3 + CH3 (1) and CH3 + CH3 (2) have been determined to be: k(1) = (2.1 +/- 0.7) x 10(-10) cm(3) molecule(-1) s(-1) and k(2) = (4.6 +/- 1.0) x 10(-11) cm(3) molecule(-1) s(-1), respectively.