A high-pressure mass spectrometer with a pulsed electron beam has been employed to determine the standard enthalpies and entropies of association of diols (D) with Cl- to form DCl- and DClD-. The -Delta H-o (kcal mol(-1)) and -Delta S-o (in parentheses, cal K-1 mol(-1)) values for the formation of DCl- are 1,2-ethanediol 25.3 (29.1); 1,3-propanediol 28.3 (34.0); 1,4-butanediol 30.2 (42.6); 2-butene-1,4-diol 32.9 (44.4); and 2-butyne-1,4-diol 28.9 (36.7). All the diols behave as bidentate ligands to give-cyclic DCl-, and the values of -Delta H-o for the C4 diols are the maximum expected when comparison is made with -Delta H-o for the formation of the Cl--bound dimers of methanol and ethanol. The effect of diol size on the magnitude of -Delta H-o is significantly less than for protonation. Although 1,2-ethanediol also has the smallest values of -Delta H-o and -Delta S-o for the formation of DClD-, the small entropy change results in this diol being the better clustering agent of all the diols at elevated temperatures.