The behavior of Os submonolayers on polycryst. Pt was studied by 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 est. the Os surface coverage. The activity of Os towards hydrogen adsorption was also studied by depositing the metal onto an Au electrode. An estn. of 40% hydrogen coverage for Os was made, which was taken into consideration in the detn. of the Os coverage. Another method for estg. the surface coverage was also considered where the oxidn. charge of OsO4 was taken as a measure of the amt. of Os present on the surface. Also the deposition potential influences the voltammetry in the hydrogen region. A potential dependent preferential deposition onto Pt and Pt surface sites was observed.