Solid state reactions of fluorine with methane have been studied by ultraviolet laser photolysis of dilute mixtures of the two reagents in solid argon at 13-30 K. Using a combination of EPR and FTIR spectroscopies, three distinct mechanisms of product formation have been identified. At temperatures below 18 K, product formation is dominated by direct photolysis of F-2-CH4 heterodimers, resulting in formation of closed-shell complexes HF-CH3F. A small fraction of reaction intermediate, a nonplanar methyl radical trapped in the reaction cage with HF and F, is also formed. This intermediate decays to product on a time scale of 10(3) s at 13 K after the photolysis period. Above 20 K, photogenerated F atoms undergo thermally activated diffusion in argon, and they react with isolated CH4 molecules to form CH3-HF complexes. The rate constant for this process is similar to 10(-25) cm(-3) s(-1) at 20 K, and exhibits an activation energy of 1.7 kcal/mol. A third reaction channel involves reaction of F with CH4 to generate isolated planar methyl radicals by escape of the HF product from the reactive site. This channel has been observed for reactions of translationally excited F atoms (below 20 K) and thermally diffusing F atoms (during post-photolysis reactions above 20 K).