A detailed analysis is presented of the Lyman‐α observations of Venus made from Mariner 5. In the analysis, we assume that the observed emission is the result of resonance scattering of the solar Lyman‐α flux by hydrogen or hydrogen and deuterium atoms in the atmosphere. The variation of the atomic densities with altitude is taken from the exospheric models of Chamberlain and McAfee. Radiative transfer effects are included but are not of crucial importance to the analysis. We find that an exospheric model consisting only of atomic hydrogen fails to give an adequate representation of the data. Instead, two constituents with mass ratios in the range 2 to 3 are required. A reasonable fit to the data is made with the hydrogen‐deuterium model after large asymmetries are introduced in the densities at the base of the exosphere. This model indicates that in the subsolar region the exospheric temperature is about 640°K and the hydrogen and deuterium densities are about 6 × 10³ and 105at 6500 km from the center of the planet. The degree of asymmetry is uncertain, but a reduction in the deuterium density by a factor of 10 and in the hydrogen density by a factor of 2 in the antisolar region at 6500 km is suggested by the night‐side