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Platinum particle growth in aqueous phase has been investigated at 20 °C in the course of nitrite (NO2−) reduction by H2 reaction. A shift from a monomodal to a bimodal particle size distribution on used catalysts is accompanied with the disappearance of the smallest Pt particles in agreement with an Ostwald ripening mechanism. A two-step process involving consecutive dissolution and re- aggregation to form larger particles has been suggested according to a redox mechanism. Accordingly, the growth of large Pt particles at the expense of small ones induces a strengthening of the metallic character and a loss of Pt dispersion.