The EUV photochemistry of methane is one of the dominant chemical processes in the upper atmospheres of Titan and Jovian planets. The dilution of CH4 with N-2 significantly changes the subsequent hydrocarbon chemistry initiated by EUV photoionization. At wavelengths below 80 rim, the presence of the dominant N-2 species in a N-2/CH4 gas mixture (=95/5) selectively enhances the formation of unsaturated hydrocarbons, such as benzene and toluene, while pure CH4 gas leads to a wide mixture of saturated/unsaturated hydrocarbon species. This enhanced formation of unsaturated hydrocarbons is most likely initiated by the generation of CH3+ via a dissociative charge-transfer reaction between N-2(+) and CH4. This mechanism was further confirmed with the dilution of CH4 with Ar, which shows similarly enhanced formation of unsaturated species from an Ar/CH4 (=95/5) gas mixture. In contrast, the depleted generation of unsaturated species from a H-2/CH4 gas mixture (=95/5) suggests that the CH5+ ion generated via a proton-transfer reaction is not an important precursor in the production of complex unsaturated hydrocarbons. Therefore, it is the dissociative charge-transfer reaction of CH4 that initiates the formation of unsaturated complex hydrocarbons through production Of C2H5+ with subsequent dissociative recombination. Implications regarding photochemistry in the upper atmospheres of Titan and the Jovian planets are discussed.