Nonthermal plasma discharge is used for the decolourization of dyes employed in textile industry. Two dyes were considered, namely methylene blue and malachite green in aqueous solution. Plasma was generated by a Pin-water surface DBD between a metal pin electrode placed in air and the surface of an aqueous dye solution filling a glass container. Active radicals, especially hydroxyl radicals, generated by plasma in air penetrated the aqueous dye solution and induced the oxidizing reactions leading to the dyeing material disintegration. The measurements of solution parameters, including pH, oxidation-reduction potential, and conductivity can help to optimize the plasma decolourization efficiency of the two dye solutions. It was found that the pH values of the two dye solutions decrease with the increasing plasma treatment time. This is accompanied by an increase of the oxidation-reduction potentials and conductivities. The concentration of hydrogen peroxide formed in the two dye solutions during plasma treatment was found to increase with the plasma treatment time. The decolourization efficiencies of the two dye solutions increase with plasma treatment time and can be related to the solution parameters, including reduction of the pH values, increase in the oxidation-reduction potentials as well as solution conductivities and increase of hydrogen peroxide during the plasma treatment time.