We have found that the intermolecular spacing of an arachidic acid (C-20) Langmuir-Blodgett (LB) film can be controlled by introducing a variable amount of spacer molecules such as fullerene (C-60) into the film. Using this characteristic, several kinds of mixed LB films, composed of C-20 and C-60, were prepared and the adsorption characteristics of gas molecules, namely phytoncid (e.g. limonene, alpha-pinene), to these films were studied using a quartz crystal microbalance. The dependence of the amount of the gas adsorbed on the number of layers reveal that the permeability of the gas molecules into the LB films remarkably increases with an increase of the intermolecular spacing of LB films. Gas molecules with different sizes or structures were sifted by using C-60 containing LB films. This characteristic of the LB films containing C-60 can be applied to a novel gas sensor with a molecular sifter function.