The early stages of Pt electrodeposition (0.675-0.620 V vs RHE) on highly oriented pyrolytic graphite (HOPG) from chloroplatinic acid at 25 °C have been studied by ex-situ STM and SEM imaging complemented with electrochemical data. Nucleation and 3D growth of Pt initiate at HOPG surface defects. Large Pt agglomerates containing flat crystallites with well-defined geometries are found around HOPG steps. Pt crystallites formed by 1-2 nm size clusters become more compact as the electrodeposition potential is shifted negatively or the Pt electrodeposited charge is increased. High-resolution STM imaging reveals large uncovered HOPG areas with the nearest-neighbor C-C distance d = 0.24 ± 0.02 nm and fiat hexagonal Pt crystallites. Electrochemical data combined with STM imaging can be interpreted in terms of a diffusion-controlled Pt(IV) to Pt(II) reaction at HOPG and a surface reaction leading to Pt(0) at HOPG defects.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA)Facultad de Ciencias Exactas