High-resolution electron spin resonance (ESR) spectra of radical pairs of a hydrogen atom that coupled with a methyl radical (H center dot center dot center dot CH3, H center dot center dot center dot CHD2, D center dot center dot center dot CH2D, and D center dot center dot center dot CD3) were observed for X-ray irradiated solid argon containing selectively deuterium-labeled methanes, CH4, CH2D2, and CD4, at 4.2 K. The double-quartet H-1-hyperfine (hf) splittings of ca. 26 and 1.16 mT at the Delta m(s) = +/- 1 and Delta m(s) = +/- 2 transitions, which are one-half of the isotropic H-1-hf splittings of an isolated H-atom and a CH3 radical, were attributed to the H center dot center dot center dot CH3 pair. The H-1-hf splittings at the Delta m(s) = +/- 1 transition were further split by the fine structure (fs) due to the electron dipole-dipole coupling. Because of the high-resolution spectra, three different sets of the fs splitting, d, are clearly resolved in the spectra of both the H center dot center dot center dot CH3 and the D center dot center dot center dot CD3 pairs. The separation distance (inter-spin distance), R, between the H-atom and the CH3 radical being in pairs was evaluated from the d values based on a point-dipole interaction model. For the case of the H center dot center dot center dot CH3 pair, the observed d values of 4.2, 4.9, and 5.1 mT yield the respective separations, R = 0.87, 0.83, and 0.82 nm, to probe the trapping site of the pair in an Ar crystalline lattice (fcc). For the pair with R = 0.87 nm, for example, we propose that the CH3 radical occupies a substitutional site and the counter H-atom occupies either the interstitial tetrahedral sites directed away from the CH3 radicals by a distance of 0.87 nm or the interstitial octahedral sites by a distance of 0.88 nm. When a mixture of CH4 and CD4 in a solid Ar matrix was irradiated, only two different radical pairs, H center dot center dot center dot CH3 and D center dot center dot center dot CD3, were observed. This result clearly demonstrates that the hydrogen atom and methyl radicals, which undergo a pairwise trapping, can originate from the same methane molecule.