Hydrogen exchange reactions between lithium and sodium compounds, MX (M = Li: X = H, CH3, NH2, OH, F; M = Na: X = CH3), and the corresponding hydrides, HX, have been modelled by means of ab initio calculations including electron correlation and zero point energy (ZPE) corrections. Small or no activation barriers (from the initial complexes) are encountered in systems involving lone pairs (10.8, 2.4, 0.0 kcal/mol for X = NH2, OH, F, respectively). Since the association energies of the initial complexes are much larger (21.0, 20.4, 23.5 kcal/mol, respectively; MP2/6-31 +G*//6-31 +G* + ZPE), such exchange reactions should occur spontaneously in the gas phase. The methyl systems (X = CH3) have the largest barriers: 26.7 (M = Li) and 31.7 ( M = Na) kcal/mol (MP2/6-31+G*//6-31G* + ZPE), and the initial complexes are only weakly bound. The significance of these systems as models for hydrogen exchange reactions in complexes of electropositive transition metals is discussed. However, the gegenion-free exchange of hydrogen between CH3-and CH, has a much lower, 11.8 kcal/mol barrier (MP2/6-31+(;*//6-31+G* + ZPE) . All the transition structures are highly ionic (charges on the metals > +0.8). The effect of aggregation has been considered by examining the hydrogen exchange between (LiX)2 and HX(X = H, CH3, NH2, OH). Although these dimer reactions formally involve six, instead of four electrons, no "aromatic" preference is observed.