Topological analysis of the electron localisation function (ELF) and the theory of atoms in molecules are applied to analyse the nature of bonds in MCCH (hl = Li, Na, K) molecules. The substitution of hydrogen by alkali metal leads to a transfer of about one electron from M to C1C2H and all alkali metal acetylides may be described by the M+ [C1C2H](-) formula. This means that the bonding between carbon and metal is mainly of ionic nature. One can explain the observed difference in properties of organosodium compounds by rather high electronegativity of sodium according to the Allred-Rochow scale.