The novel features in the sulphonation of aryl gp.-contg. mono-, di-, oligo- or poly-phosphines with oleum to give sulphonated arylphosphines are that the temp. is 0-80 (pref. 10-50) deg.C and Lewis acids are present. Pref. sulphonation is effected by adding oleum to a soln. of arylphosphine and Lewis acid (esp. boric acid) in conc. H2SO4, the soln. esp. being satd. with boric acid.