We report here that ultrasound (20 kHz and 40 kHz) affects the catalyst ink composition when irradiated for longer periods and at high ultrasonic powers. In our study, two commercial carbon supported Pt (Pt/C) catalysts were used and dispersed in Nafion (R) ionomer. Catalyst ink samples prepared from Nafion (R), IPA and water were either ultrasonicated (20 kHz up to 12.23W and 40 kHz at 1.82W) or mechanically shear-mixed (19,000 rpm) for various durations (up to 120 min). All catalyst ink samples were characterised by XRD, BET, TEM and electrochemical measurements were performed in liquid electrolytes. It was found that an optimised ultrasonic treatment is required to improve the catalytic ink activity, but longer irradiation is detrimental to its composition and morphology, mainly due to cavitation and sonolysis phenomena. (C) 2014 Elsevier Ltd. All rights reserved.