Continuous sulphonation of toluene by gaseous sulphur trioxide has been studied in a pilot - scale jet loop reactor with a external heat exchanger. The experimental results indicate that the isomer distribution of toluene sulphonic acids is unaffected by other operating conditions under a given temperature. The content of meta isomer in the mixture of isomers is Jess than 1.2% .whereas that of para isomer is around 85% under the reaction temperature of 10 ℃ and conversion of toluene in the range of 30% - 40%.