PURPOSE: To reduce the size of the pre- and the post-treatment facilities in the counter- current and continuous sulfonation of an organic compound with an SO3-containing gas, and to obtain the high quality objective product in high degree of conversion, by introducing the raw material compound to the waste gas produced by the reaction, and circulating the mixture of gas and liquid with a liquid ring pump.

CONSTITUTION: An organic compound is sulfonated countercurrently and continuously in the first reactor 2 with an SO3-containing gas sent through the pipe 13, and the sulfonated product separated by the vapor-liquid separtor 3 is taken out through the pipe 15. A part of the waste gas separated in the separator is exhausted through the pipe 12 and the waste gas treatment apparatus. The remaining part of the waste gas circulating through the pipe 11 is mixed with the organic compound raw material 4 sent through the pipe 14, and the organic compound and the waste gas are circulated in the form of a vapor-liquid mixture with a liquid ring pump 1. Pref. the second reactor 2' is placed at the upstream of the liquid ring pump 1, and a part of the SO3- containing gas is supplied to the reactor through the pipe 13'. Generation of waste gas mist in the recycling tube, carbonization of residence liquid, and accumulation of carbonized material can be prevented by this process.