Sulfonation process

In the process for the sulfonation of organic compounds by contacting the organic compounds with a sulfonating agent selected from sulfur trioxide, a solution of sulfur trioxide in sulfuric acid and chlorosulfonic acid, the improvement comprising maintaining the sulfonating agent and the organic compound in a substantially non-aqueous environment and substantially separated by means of an acid polymer membrane which is substantially chemically inert to the action of both the organic compound and the sulfonating agent and which in said substantially non-aqueous environment is substantially impermeable to said organic compound and substantially permeable to said sulfonating agent.