Ultraviolet photolysis of CD3I in solid parahydrogen at 5 K gives CD3 radical, which decreases in a single exponential manner with a rate constant of (4.7 +/- 0.5) x 10(-6) s(-1). Concomitantly, CD3H is formed, which is accounted fur by the quantum tunneling reaction CD3 + H-2 --> CD3H + H. Under the same conditions, CH3I yields CH3 radical, hut the corresponding reaction between CH3 and H-2, expected to give CH4 + H, does not proceed measurably at 5 K. The difference between the two systems is attributed to the difference in the zero point energy change.