The CH2((X) over tilde(3)B(1)) + H --> CH(X(2)II) + H-2 reaction (1) was investigated in CH2CO/O systems (T = 400, 500, 650, and 950 K, p = 2 Torr) as well as in C2H2/O systems (T = 590 and 890 K, p = 4 Torr) using discharge-flow/molecular beam sampling mass spectrometry techniques (D-F/MBMS). The first rate coefficient data at temperatures intermediate between room and flame temperature are presented. The CH2((X) over tilde(3)B(1)) + H rate constant was measured relative to the well-known k(CH2((X) over tilde(3)B(1)) + 0) = (1.3 +/- 0.3) X 10(-10) cm(3) molecule(-1) s(-1) from the change of the quasi-steady state CH2 concentration upon varying the [H]/[O] ratio at given CH2 formation rate. In the temperature range of interest, the k(1) coefficient was found to exhibit a clear-cut negative temperature dependence. The following k(1) values were obtained : (i) using the CH2CO + O reaction as CH2 source, (2.93 +/- 0.80) x 10(-10) at 400 K, (2.05 +/- 0.57) x 10(-10) at 500 K, (1.22 +/- 0.31) x 10(-10) at 650 K, and (1.18 +/- 0.32) x 10(-10) at 950 K; (ii) using the C2H2 + O system as CH2 source, (1.90 +/- 0.58) x 10(-10) at 590 K and (1.12 +/- 0.36) x 10(-10) at 890 K (k in cm(3) molecule(-1) s(-1); 95% confidence intervals, including possible systematic errors). The data obtained with the two CH2 sources are in good mutual agreement. The decrease of k(1) with temperature is in accord with literature k(1) data at room temperature on one side and in the 1500-2500 K range on the other. However, the observed temperature dependence in the 300-1000 K range is much less steep than predicted by the recommended k(1)(T) expression in a recent evaluation.(44) An equation that fits all available data reasonably well is k(1) = 3.8 x 10(-10) exp [-1.3 x 10(-3)T(K)] cm(3) molecule(-1) s(-1), for T = 300-2500 K.