Methane was irradiated with microwave operated helium (21.2 eV) and neon (16.7-16.8 eV) resonance lamps whi ch were separated from the reaction vessel by an aluminum window. The quantum yields of the stable end products hav e been dete rmin ed at methane pressures ranging from 1 to 20 torr. Over thi s pressure ran ge the ab und ances of the primary ions, dete rmined throu gh various diagnostic expe riments, are within experim ental error the sa me at 21.2 eV as at 16_7 -16.8 eV ItC H; = CH t = 0.5, CH:; ~ 0.02), and are in good agree ment with th e primary mass s pectra obtained in a photoionization mass s pectrometer und er collision free condition s (P < 10-5 torr). The C,Ht whi ch is form ed by the reaction: CHt + CH. ~ C,Ht + Hz loses a proton by an undetermined mec hanism to give C,H. as a product.There is no evid ence for the formation of neutral fragm ents such as H2 , C, CH or CH, at 16.7-U,,8 eV. The fact that the ionization quantum is equal to unity in thi s e nergy range accounts for the absence of these intermediates. At 21.2 eV wh ere (