Highly {100} oriented Pt deposits were prepared by electrodeposition from a 10 mM HCl, 100 mM KCl and Na2PtCl6 center dot xH2O electrolyte. The deposits were prepared in the form of thin films and array of nanowires. A qualitative assessment of the proportion of {100} oriented Pt surfaces was obtained through X-ray diffraction measurements and cyclic voltammetry in 0.5 M H2SO4. The effect of the deposition potential, Edep, temperature of the electrolyte, Tdep, platinum salt concentration [Na2PtCl6.xH2O], and nature of the substrate were investigated. It was shown that the proportion of {100} oriented Pt surfaces reaches a maximum for Edep = -0.35 V vs SCE. Moreover, this proportion increases steadily as Tdep and [Na2PtCl6 center dot xH2O] are decreased from 75 to 25 degrees C and from 2.5 to 0.25 mM, respectively. Scanning electron microscopy and high-resolution transmission electron microscopy micrographs indicate that the more oriented samples are made of pine tree-like structures that are effectively single crystals, and that the growth facets appear to be close to the {001} plane. This observation also clearly indicates that the plane exposed during the CV experiment is also {001}. As suggested by these micrographs, the films and nanowires are highly porous and roughness factors as large as 1000 were obtained on highly {100} oriented Pt nanowires. The predominance of {100} facets is attributed to their energetically favoured growth in the presence of hydrogen, and is shown to be significantly enhanced when the mass transport of Pt4+ is limited. Due to the predominance of {100} facets, the normalized electrocatalytic activity (mu A cm-2Pt) for the electro-oxidation of hydrazine and ammonia is higher than non-oriented polycrystalline Pt by a factor of 4 and 2.7, respectively.