Abstract: The coordination reactions of 2,3-benzo-11-methyl-18-crown-6(BCl-18C6) with eight alkali and alkaline earth (Na,K,Rb,Cs, Mg, Ca,Sr,Ba) chlorides were determined at 30℃ and in 70% W/Wmethanol by self-made titralion calorimeter. The stability constants of coordination compounds and the enthalpy of coordination reaction were calculated directly. Because the space hindrance gives a weaker effect to small metal ions than to large ones, and the space hindrar.ce of crown with only one methyl is weak, the stability sequence of alkali metal coordination compounds is K> Rb>Na≥Cs and is different from that of 2, 3-benzo-8, 11, l5-trimethyl-18-crown-6 (BC3-18C6). Furthermore, the stability constants of (BC3-18C6) coordinition compounds with Mg and Ca are even higher than those of BC1-18C6 pnes because of the small radii of Mg and Ca.