Lipase from Burkholderia cepacia (lipase BC) and Lipase B from Candida antartica (CALB) show an increase of the transesterification activity in toluene (up to 2.- and 1.7-fold, respectively), when lyophilized with 18-crown-6. Nevertheless, the increase was observed only for low (less than 100) 18-crown-6/lipase molar ratio, while at higher ratios, the activity decreased for both enzymes to values lower than those obtained in the absence of the addictive. In 1,4-dioxane, the activation is lower for lipase BC (1.7-fold) and for CALB (1.5-fold). Concerning enantioselectivity, tested in the kinetic resolution of 6-methyl-5-hepten-2-ol, only in the case of CALB, aneffect of the addictive (the E value varied from about 120 to 128) was observed. In water, 4% (w/w) of 18-crown-6 caused a loss of activity in the hydrolysis of p-nitrophenyl laurate of about 88 and 99.75%, compared to that observed in the absence of the crown ether for CALB and lipase BC, respectively. These data nad the conformational analysis of both lipases, carried out by FT/IR spectroscopy indicate that the enzyme inactivation in water and in organic solvents at 18-crown-6/lipase molar ratios, higher than 100 might be due to conformational changes caused by the addictive. Instead, at molar ratios lower than 100, 18-crown-6 might increase the activity-particularly, in tolune-thanks to the fact that in its presence, the enzyme has an hydrogenbonds pattern, more similar to that in water. This suggests that the additive would be able to provide the enzyme with more water.