Toluene was sulfonated by gaseous sulfur trioxide to prepare p-toluenesulfonic acid. The effects of reaction temperature and toluene conversion on the isomer selectivities and the dynamic viscosity of the reaction mixture in a well-stirred tubular glass reactor of 4cm diameter and 16 cm height were investigated. Lower selectivity to the m-isomer was obtained at lower reaction temperature and lower conversion of toluene, whereas the selectivity to the p-isomer seemed to be less affected by these parameters. With the increase in toluene conversion a sharp increase in viscosity of the reaction mixture was observed, which was attributed to the saturation of the toluenesulfonic acids dissolved in toluene. A pilot-scale stainless steel jet loop reactor (JLR) of 0.3 m diameter and 4m height with an external cooler was constructed and used for the continuous sulfonation of toluene. It was found that the JLR was very effective for the fast removal of the large amount of reaction heat, and selectivities to the p-isomer around 85% and to the m-isomer below 1.2% were obtained.