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A simple and direct gas chromatography–mass spectrometry method for routine estimation of C2 H5-alkylbenzenes (or C2-alkylbenzenes), namely, ethylbenzene and m / p - and o -xylenes in the boiling range of 80°C–400°C, including diesel range streams using deuterated ethylbenzene as internal standard has been described. Estimation of C2-alkylbenzenes in feeds and products is important for monitoring the reaction kinetics in certain secondary processes which involve conversion of low value, aromatic-rich diesel range boiling streams to produce aromatic-based petrochemical feedstocks upto xylenes. The method developed takes advantage of chromatographic separation and resolution masses of fragment ions of the compounds of interest for estimation. Linearity in estimation has been established in the range of 300 ppm to 5% (w/w) for ethylbenzene, m / p -xylenes and o -xylenes. Correlation coefficient (R 2) value of 0.99 was obtained for the three compounds for calibration mixtures. Total 15 samples obtained from refineries, pilot plant for mono-aromatic enrichment studies, and commercial diesel samples were analyzed by this method for estimation of ethylbenzene, m / p- and o -xylenes content. [ABSTRACT FROM AUTHOR]