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The relationships between physicochemical param- eters of majority components of Eucalyptus essential oils and their insecticide effect were evaluated on Aedes aegypti (L.) (Diptera: Culicidae). The octanol?water partition coefficients of the monoterpenes were estimated by the atom/fragment contribution method and the vapor pressures were determined by our laboratory in previous studies. The larvicidal activity (LC50 (ppm)) and knockdown effect (KT50 (min)) of each com- ponent was determined. The results show that the toxicity of EOs main components of Eucalyptus on adults and larvae of A. aegypti is strongly related to their physicochemical prop- erties (vapor pressure and Log P). However, the interaction of both variables (vapor pressure * Log P) explains the toxico- logical phenomenon more precisely. The regression models were expressed as follows: KT50(min)=!10.9+3.7*Log P+ 1.9*1/Pvapor (R2=0.80;F=42.5) and LC50(ppm)=!94.3+ 438.6* 1/Log P+2.8* 1/Pvapor (F=57.8;R2=0.85). The six evaluated components present different functional groups. Therefore, it was considered to evaluate the monoterpenes as a group and separated in two groups: oxygenated monoter- penes (!-terpineol, 4-terpineol, and 1,8-cineole) and terpene hydrocarbons ( -terpinene, p-cymene, and !-pinene). The results show the regression models for each group as follows: (A) oxygenated terpenes: KT50(min)=!515.3+1613.2"1/ Log P+5,2"1/Pvapor (F=3176.7R2=0.99) and LC50(ppm) =! 1679.4+5402.1"1/Log P+12.7" 1/Pvapor (F=282.9;R2 = 0.99). (B) Hydrocarbons terpenes: KT50(min)=18.2!58.3*1/ Log P+2.7*1/Pvapor (F=171.7; R2=0.97) and LC50(ppm)=! 21.1+174.9"1/Log P!14.3"1/Pvapor (F=410.0;R2 =0.99). The association between the toxic effect of the evaluated monoterpenes against A. aegypti and the physicochemical properties can be better described when they are separated into functional groups (hydrocarbons vs. oxygenated terpenes).