The reaction, CD3+ Me4C → CD3H + Me3CCH2(1), has been re-investigated by the photolysis of hexadeuterioacetone in the presence of neopentane. Between 185 and 309° the rate constant was found from log k1(mole–1 ml. sec.–1)= 12·3 –(12,000/2·3RT). The Arrhenius parameters are in good agreement with results of other hydrogen abstractions by methyl radicals from alkanes, obtained by the same method. The activation energy obeys the Polanyi relation, E= 0·49 [D(C–H)– 74·3] on the basis of D(Me3CCH2–H)= 99 kcal. mole–1.