The present invention relates to a preparation process for 2-arylbenzimidazolesulfonic acids of the formula I: ##STR1## Ar is a substituted or unsubstituted phenyl or naphthyl radical and R is a C.sub.1-8 -alkyl or C.sub.1-8 -alkoxy radical. n is 1-4, m is 1-3 and o is 0-2. o-phenylenediamine is reacted in the presence of oleum with an arylcarboxylic acid or an arylcarboxylic acid derivative. Compounds prepared in this way can be used as UV filters, and as components in cosmetic compositions.