The behaviour of Os submonolayers on polycrystalline Pt was investigated by means of a flow cell of impinging jet type. Os was electrodeposited onto the electrode for various periods of time. From the loss in the hydrogen region charge it was possible to estimate the Os surface coverage. The activity of Os towards hydrogen adsorption was also studied by depositing the metal onto an An electrode. An estimation of 40% hydrogen coverage for Os was made, which was taken into consideration in the determination of the Os coverage. Another method for estimating the surface coverage was also considered where the oxidation charge of OsO4 was taken as a measure of the amount of Os present on the surface. It was also found that the deposition potential influences the voltammetry in the hydrogen region. A potential dependent preferential deposition onto Pt[110] and Pt[100] surface sites was observed.