Abstract: Gas chromatography was applied to the determination of thymol in the products of isopropylation of m-cresol.Good separation was attained under the following optimum conditions:① chromatographic column used:DB-1 capillary column;② column temperature:175 ℃;③ temperature of vaporizer:210 ℃;④ tempurature of FID detector:250 ℃;⑤ internal standard used:toluene.Linear relationship between values of ratio of peak area of thymol to toluene (ATML/ATLN) and mass concentration of thymol was obtained in the range within 200 g·L-1.Recovery was tested by standard addition method,and value of average recovery found was 100.6% with value of RSD (n=7) of 1.31%.As shown by the yieid of thymol calculated from the data of determination,the gas phase catalysis using γ-Al2O3 as catalyst gave highest efficiency for isopropylation of m-cresol.