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An underwater dielectric barrier discharge (DBD) plasma jet excited by a repetitively pulsed nanosecond pulsed power modulator was used to study plasma-induced decolorization of a 1.4 × 10-4 M solution of methylene blue (MB). Past plasma decolorization studies have focused on spectrophotometry as the main diagnostic to assess decomposition. Because spectrophotometry is a measure of changes in the structure of molecular color, it is not an absolute measure of decomposition. In this paper, high-pressure liquid chromatography is used in parallel with spectrophotometry for comparison purposes and to assess the degree of true decomposition. Spectrophotometry results were found to be in agreement with chromatography measurements, suggesting that, at least in the case of MB, spectrophotometry is an adequate measure of decomposition. Additionally, the utility of a dual plasma jet applicator for rapid contaminant decomposition was explored.