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Summary form only given. Water pollution by organic dyes presents an imminent environmental issue. In this study, the degradation of aqueous solutions containing azo dyes (methyl orange and methyl red) was carried out by means of atmospheric plasma technique. As indicated by optical emission spectroscopy (OES), the metastable argon (Ar*) in the atmospheric discharge was responsible for generating reactive species such as hydroxyl (OH) radicals in aqueous solutions. The bleaching of solution and degradation of solute was the indication of destruction of dye molecules by plasma. We found that the decomposition pathways critically depend on the pH values in aqueous solutions. The plasma degradation of dyes is found to be far more efficient in neutral condition (pH ≅ 7) than in acidic conditions (pH <; 7). The degradation mechanisms of azo dyes in aqueous solutions are proposed based on uv-visible absorption spectroscopic and liquid chromatography-mass spectrometry (LC-MS) measurements.