Isothermal sulphonation process, by contacting a falling thin film-like stream (I) of one or more organic cpds. with a parallel-streaming SO3/inert gas mixt. (II) takes place by allowing an SO3-contg. cooling gas (III) contg. 1-20 wt. % of total SO3 required in process, to flow between (I) and (II). Quantity of (III) is 2-12 (2-7) x vol. of (II). The process gives high degrees of conversion, leading to formation of high-quality prods. free from coloured by-prods. and requiring no further bleaching. Mist in waste gases is reduced. Temp. of (III) is 10-40 degrees C and velocity of (III) may correspond to that of (II) or reach 1.4 x that of (II). (II) is introduced at 20-100 (30-80) m/sec. at 20-50 degrees C SO3 concn. in (III) is 0.01-2 (0.02-1) vol. % and SO3 concn. in (II) is 3-15 vol. %. (III) advantageously also flows parallel to (I) and (II).
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