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Fine (<2.6 μm) and size-segregated aerosol samples collected at a tropical pasture site (Rondoˆnia, Brazil) under various conditions have been analysed by thermally assisted hydrolysis and alkylation, coupled to GC/MS. Fatty acids, alkadioic acids, hydroxybenzoic acids, levoglucosan, and other compounds of polysaccharide origin have been identified as their methyl ester and ether derivatives among the products of thermally assisted methylation. The identity of the characteristic hydrolysed fragments and soluble components has been confirmed by butylation and silylation as well. To improve available chemical information, this analytical method has also been combined with a thermal pre-treatment. Seasonal variation of size-resolved distribution of the characteristic fragments of thermally assisted hydrolysis and methylation (THM) in the atmospheric aerosol is shown. Although the results presented in this paper are preliminary and based on a limited number of aerosol samples, they clearly demonstrate the sensitivity and potential of thermally assisted alkylation-GC/MS to provide unique chemical information on the bulk of organic matter in tropospheric aerosol to facilitate their source apportionment. [Copyright &y& Elsevier]