Nitric oxide adlayers formed at Pt(100) electrode surfaces have been characterized in situ by means of electrochemical and FTIR spectroscopic experiments. These adlayers can be reductively stripped from the electrode surface yielding dissolved ammonium as the main reduction product in acidic solution. From the voltammetric charge involved in this process the absolute NO coverage for the saturated adlayers has been calculated to be 0.5. This value agrees with that corresponding to the c(4 x 2) overlayer found by different authors under UHV conditions. The oxidation of the NO adlayer in acid media proceeds between 0.90 and 1.10 V RHE through the formation of surface species (probably a mixture of nitrite and nitro species), which can be further oxidized to dissolved nitrate anions at higher potentials. The same NO adlayers have been found to oxidize directly to dissolved nitrite anions in alkaline solutions. The potential dependence of the N-O stretching mode, which is shifted towards higher wavenumbers when the electrode potential increases, has been analyzed for the saturated NO adlayer. Linear relations were found both in acidic and neutral solutions with tuning rates of ca. 59 cm(-1) V-1 in the potential region between 0.50 and 0.90 V RHE where the saturated NO adlayers are stable. A slow dissolution process has been evidenced at potentials slightly below 0.80 V for the NO adlayer in alkaline solutions.