Methyl fluoride and fluoroform have been irradiated with gamma rays from a 1200-Ci60Co source. Ionization currents were measured using a Pyrex parallel-plate-type ionization chamber.Wvalues of 27.2 and 28.0 eV were determined for the two gases, CH3F and CHF3, respectively. The method employed was a comparative one, making use of the Bragg–Gray theory of cavity ionization.G(H2) yields of 4.20 and 1.25 for CH3F and CHF3, respectively, were then calculated from the aboveWvalues and the measured ion-pair yields for hydrogen. The formation of hydrogen is probably due to the fact that the C—H bonds in these compounds are weaker than the C—F bonds.