For the first time, this study describes the sonoelectrochemical synthesis of Pt nanoparticles from solutions containing Pt salt, ionomer, solvent and water in the presence of ultrasound (20kHz,~100W.cm^-2) using a specially designed/constructed Glassy Carbon (GC) electrode acting as a Vibrating Working Electrode (VWE) [Fig.1]. From early studies, it was found that Pt mean nanoparticle sizes could be produced in the range [11 - 15nm] in the absence of surfactants and solvents [1,2]. In this investigation, the syntheses of Pt nanoparticles were performed galvanostatically at (298±1)K, at various ultrasonic and current pulses using several time managements and at a high cathode efficiency.  Physical characterisations by TEM, SEM, XRD and SAED showed that Pt mean grain sizes and cluster sizes of up to ~20nm and ~100nn were obtained respectively. Non-ultrasonicated (silent) but high-shear mixed catalyst inks made of Pt/C(Vulcan XC-72R)/Nafion®were prepared using the method described by Curnicket al. [3,4]. Electrochemical characterisation (Cyclic and RDE voltammetry) data of the as-prepared catalyst inks showed low ECSA values (<~35 m².g_Pt^-1). This finding was mainly attributed to: (i) the fairly ‘large’ Pt nanoparticle/cluster sizes, (ii) the ionomer acting as a ‘surfactant’ encapsulating the Pt nanoparticles thus preventing local electron transfer and (iii) poor dispersion of Pt nanoparticles/homogeneisation of the ink caused by the high-shear mixer (19,000rpm).  However it was observed that the ionomer prevented Pt nanoparticles aggregation. It was suggested that the Nafion® content may play an important role on the activity of the Pt nanoparticles as previously observed [3,4]. It also possible that ultrasonication affects the composition and morphology of the electroanalyte/Pt nanoparticles/ionomer, mainly due to cavitation and sonolysis phenomena as recently shown by Pollet and Goh [5,6].