We studied the surface properties of indium-tin oxide (ITO) modified by wet (aquaregia, ultrasonication, RCA) and dry (oxygen- and argon-plasma) treatments. The surface modification was investigated by surface energy, surface morphology, sheet resistance, carrier concentration, carrier mobility, and workfunction measurements. We report that the studied oxygen-plasma treatment induces: the highest surface energy with the highest polarity, the smoothest surface, the highest carrier density but the lowest mobility, the lowest sheet resistance, and the highest workfunction (stable in air). Polymer light-emitting diodes fabricated with the oxygen plasma treated substrates give the best performance in terms of electroluminescence efficiency and device lifetime. This is attributed to a favorable surface modification of ITO anodes by oxygen-plasma.