A gas-liquid plasma reactor is used as an Advanced Oxidation Process for water treatment. The plasma discharge is carried out at atmospheric pressure using AC high voltage and different gases (N-2, O-2, Ar) or gas mixtures. The originality of this work lies in the measurement of the number, frequency and mean energy of micro-discharges under the different gaseous atmospheres using iCCD camera visualization. Treatments in argon present the best results. This may be due either to the positive role of metastable argon in the degradation of the pollutant and/or to the way the oxidant species (HO center dot radicals) are brought to the solution at the liquid-plasma interface. In fact, unlike discharge in oxygen, discharge in argon is characterized by low energy micro-discharges evenly distributed in time. (C) 2014 Elsevier B.V. All rights reserved.