Liquid formulations of basic azo dyes are prepared from unsubstituted or C.sub.1 -C.sub.4 -alkyl- or C.sub.1 -C.sub.4 -alkoxy-substituted m-phenylenediamine by diazotizing and coupling in carboxylic acid solution, which comprises performing the reaction with 0.76 to 0.95 mole of nitrile, calculated as nitrous acid and based on 1 mole of m-phenylenediamine, and then heating the resulting reaction mixture at from 60.degree. C. to the boiling point of the reaction mixture.