The structure, stability and reactivity of HX⋯H 2 and HXCH 2 ⋯H 2 (X = O, S) were studied using standard ab-initio MO calculations including electron correlation (MP2, MP3, PMP3, MP4). In the most stable conformation of HX⋯H 2 , the H-X bond is antiparallel to one α-CH bond. The predicted height for the rotational barrier around the HX-·CH 2 bond is ca. 4 kcal mol t1 for both radicals (X = O, S). The inversion barrier around the radical centre is weak in HO⋯H 2 (⋍ 1 kcal mol −1 ) and becomes negligible in HS⋯H 2 . The best conformation around the HX-CH 2 ⋯H 2 bond is the gauche one for both radicals (X = O, S), but the best conformation around C-C is reached when the SOMO is antiparallel to one /gb-CH if X = O, or antiparallel to C-X if X = S. HOCH 2 ⋯H 2 is slightly more stable than HS⋯H 2 , but CH 3 S . is much more stable than CH 3 O . . HOCH 2 ⋯H 2 is slightly less stable than HSCH 2 ⋯H 2 . The easy 1,2-migration of HS in HSCH 2 ⋯H 2 clearly depends on the weak bond-dissociation energy of HS-CH 2 ⋯H 2 . A direct 1,2-H shift in HX⋯H 2 is energetically costly.