The effect of methylation on gas sensing properties of beta-Cyclodextrins was studied with the piezoelectric quartz crystal microbalance technique. Molar adsorptivities of three beta-Cyclodextrins (un-methylated, partially methylated, and fully methylated) towards alcohols and benzene derivatives were measured and compared, and a two-step sorption model was used to elucidate the nature of interactions between beta-Cyclodextrins and the organic vapor molecules. For the un-methylated beta-Cyclodextrin, the inclusion of alcohols is the greatest for methanol, followed by ethanol and n-propanol, in parallel with the hydrogen bonding acidity of the alcohol. Methylation reduces the hydrogen bonding and the magnitude of the acidity effect. Methylation also increases the van der Waal's force, which exerts a reverse effect and makes A(cv) slightly increasing from methanol to n-propanol. For benzene derivatives, the inclusion is the least for m-xylene, followed by toluene and benzene for the un-methylated beta-Cyclodextrin, due to the molecular size effect. However, the van der Waal's force exerts a reverse trend, and becomes more important with methylation of the beta-Cyclodextrin. The balance between various interactions plays an important role in the sensitivity and the selectivity of beta-Cyclodextrin sensors.