The invention concerns a method for preparing polyisocyanates by cyclodimerization of isocyanate functions borne by initial monomer isocyanates. The invention is characterised in that it comprises steps which consist in: a) providing a reaction medium comprising initial monomer isocyanates, optionally in the presence of a solvent; b) adding to said reaction mixture a (cyclo)dimerization catalyst comprising a compound of the family of super acids; c) heating the reaction medium to a temperature ranging between 0.degree. C. and 300.degree. C., advantageously between 20.degree. C. and 200.degree. C. until the desired rate of transformation is obtained; d) optionally inactivating or eliminating the reaction catalyst; and e) optionally eliminating the unreacted monomer.