The degradation of aqueous solutions containing azo dyes (ortho-, meta-, and para-methyl red) was carried out by means of atmospheric pressure plasma treatment. As evidenced by optical emission spectroscopy, the metastable argon in the discharge is responsible for initiating reactions in dye solutions. The bleaching of aqueous solutions is attributed to the destruction of dye molecules as observed in the UV-visible absorption spectra. We found that the degradation pathways of methyl red critically depend on the pH values in aqueous solutions as well as isomeric structures. The reaction pathways are entirely different in basic (pH = 11), near-neutral (pH = 6), and acidic conditions (pH = 2). Kinetic analysis shows that acidic condition gives the fastest degradation rates of methyl red isomers with removal rate: ortho > meta > para among all conditions. At basic condition, the degradation rates are equally slow for all methyl red isomers.