If highly concentrated aqueous solutions of azo dyes are prepared directly by diazotization and coupling reactions, appreciable difficulties generally result as a consequence of the fact that the diazotization and coupling reactions give rise to viscous phases which lead to stirring problems in the reaction apparatus used, which in turn results in an incomplete synthesis. The novel process for preparing concentrated aqueous dye solutions of an anionic dye in an aqueous medium overcomes this disadvantage by carrying out the diazotization of the aromatic amine diazo component and/or the coupling reaction between the diazonium salt and the coupling component in the presence of an auxiliary which exerts a viscosity-reducing effect on pasty phases. Examples of such auxiliaries are optionally alkyl-substituted naphthalenesulfonic acid-formaldehyde condensation products, ligninsulfonates, polyacrylates and copolymers of maleic anhydride and methacrylic acid or acrylic acid.