The goals of this research were to i) test the ability of air non-thermal plasma to promote the degradation in water of two widespread herbicides, mesotrione and metolachlor, ii) determine the process efficiency and products for each herbicide treated individually, iii) detect and investigate any possible reciprocal effects when the two herbicides, which are often used in combination, are treated together in the same solution. The experiments were carried out using a dielectric barrier discharge (DBD) reactor to generate a strongly oxidizing non-thermal plasma above the liquid surface and induce the advanced oxidation of dissolved organic chemicals. The results of extensive HPLC (UV, MS and MS/MS) and TOC analyses show that air non-thermal plasma efficiently decomposes both herbicides, that mesotrione is considerably more reactive than metolachlor and that complete mineralization of both is achieved at sufficiently long treatment times. Treatment of metolachlor was also performed in tap water, a closer model for environmental systems, achieving the same results as found in deionized water. Detailed product and kinetic studies failed to reveal any effect attributable to reciprocal influence of mesotrione and metolachlor when treated together in the same solution.