The reactions of singlet methylene (a(1)A(1) (CH2)-C-1) with hydrogen and deuterium have been studied by experimental and theoretical techniques. The rate coefficients for the removal of singlet methylene with H-2 (k(1)) and D-2 (k(2)) have been measured from 195 to 798 K and are essentially temperature-independent with values of k(1) = (10.48 +/- 0.32) x 10(-11) cm(3) molecule(-1) s(-1) and k(2) = (5.98 +/- 0.34) x 10(-11) cm(3) niolecule(-1) s(-1), where the errors represent 2 sigma, giving a ratio of k(1)/k(2) = 1.75 +/- 0.11. In the reaction with H2, singlet methylene can be removed by reaction giving CH3 + H or deactivated to ground-state triplet methylene. Direct measurement of the H atom product showed that the fraction of relaxation decreased from 0.3 at 195 K to essentially zero at 398 K. For the reaction with deuterium, either H or D may be eliminated. Experimentally, the H:D ratio was determined to be 1.8 +/- 0.5 over the range 195-398 K. Theoretically, the reaction kinetics has been predicted with variable reaction coordinate transition state theory and with rigid-body trajectory simulations employing various high-level, ab initio-determined potential energy surfaces. The magnitudes of the calculated rate coefficients are in agreement with experiment, but the calculations show a significant negative temperature dependence that is not observed in the experimental results. The calculated and experimental H to D ratios from the reaction of singlet methylene with D-2 are in good agreement, suggesting that the reaction proceeds entirely through the formation of a long-lived methane intermediate with a statistical distribution of energy.