The reactions of atomic carbon in its first excited D-1 state with both CH4 and C2H6 have been investigated using a continuous supersonic flow reactor over the 50-296 K temperature range. C(D-1) atoms were generated in situ by the pulsed laser photolysis of CBr4 at 266 nm. To follow the reaction kinetics, product H atoms were detected by vacuum ultraviolet laser-induced fluorescence at 121.567 nm. Absolute H-atom yields for both reactions were determined by comparison with the H-atom signal generated by the reference C(D-1) + H-2 reaction. Although the rate constant for the C(D-1) + CH4 reaction is in excellent agreement with earlier work at room temperature, this process displays a surprising reactivity increase below 100 K. In contrast, the reactivity of the C(D-1) + C2H6 system decreases as the temperature falls, obeying a capture-type rate law. The H-atom product yields of the C(D-1) + CH4 reaction agree with the results of earlier crossed-beam experiments at higher collision energy. Although no previous data is available on the product channels of the C(D-1) + C2H6 reaction; comparison with earlier work involving the same singlet C3H6 potential energy surface allows us to draw conclusions from the measured H-atom yields.