The denitration by HCO2H of highly concentrated HNO3 media (3-5 M) was studied on PtSiO2 catalysts with Pt particle size varying from 1 to 10 nm. The reaction was carried out in batch mode at 343 K. The actual active intermediate is HNO2, which is formed very slowly in the absence of Pt/SiO2. The chief function of Pt/SiO2 is the initial fast generation of HNO2 in order to reach a threshold concentration of 5 x 10(-3) mol l(-1). Above this threshold, the homogeneous process between HNO3 and HCO2H, with HNO2 as an autocatalytic species, prevails. The initial generation of HNO3 on Pt/SiO2 is regulated by a catalytic cycle involving "Pt-H" and Pt-0 species. This was demonstrated by redox cycle experiments in which Pt-H was reacted with HNO3 to yield Pt-0, which was back reactivated to Pt-H by HCO2H. Large Pt clusters were the most active, possibly due to a faster reaction rate between Pt-H and HNO3. (C) 2003 Elsevier B.V. All rights reserved.