Abstract: Mesitylene and its objective oxidation product in liquid phase,trimesic acid,were determined by gas chromatography (GC) with capillary column.To overcome the difficulty of high boiling point of trimesic acid in GC determination,it was esterified with methanol using conc.H2SO4 as catalyst,to give the corresponding carboxylate which was then determined by GC.iso-Propyl benzene and benzyl benzoate were used as internal standards in the determination of mesitylene and trimesic acid respectively.FID detection was used in the determination.Precision of the method was tested by analyzing 3 known samples of mesitylene and sym-trimethyl-benzene tricarboxylate with 5 determinations for each sample,giving values of RSD′s 0.68% and 2.84% respectively.Test for recovery of mesitylene,sym-trimethyl-benzene tricarboxylate and trimesic acid was also made,giving results in the ranges of 97.72%-104.27%,98.41%-104.72% and 98.38%-102.51% respectively.