Methane can be converted into alkanes (from C-2 to C-6) continuously by ultraviolet (185 nm) irradiation in the presence of water vapor. The products from this reaction are alkanes, which is different from the comparable heterogeneous catalytic reactions, where alkene formation is also observed. The mechanism involves the coupling of alkyl radicals formed by hydrogen abstraction with OH radicals produced following the UV irradiation of water.