We isolate an [O.CH4](.-) intermediate in the reactive O.- + CH4 encounter using an argon cluster-mediated trapping technique and characterize it using vibrational predissociation spectroscopy. The spectra of the argon-solvated complexes establish that only the OH-.CH3. ion-radical adduct is prepared. Its formation is firmly established by the appearance of the signature OH- stretching band close to that of the free hydroxide ion. The band origin locations and partially resolved rotational spacings indicate that hydroxide binds onto one of the methyl hydrogen atoms, much like the motif observed previously in the I-.HCH2. ion-radical complex. This OH-.CH3. species is best regarded as an entrance-channel complex in the secondary (endothermic) OH- + CH3. --> H2O + CH2- proton transfer reaction. These observations indicate that the initial H-atom abstraction step (O.- + CH4 --> OH- + CH3.) occurs too quickly to enable capture of the intermediates directly associated with this process.