Comprehensive model calculations of the dayside ion density distributions were carried out and compared with results from the Pioneer Venus ion mass spectrometer. The coupled continuity and momentum equations were solved for O2+, O+, CO2+, C+, N+, He+, and H+ densities for altitudes well away from the ionopause, where the horizontal transport terms are negligible. Chemical equilibrium solutions, describing conditions below about 200 km, were also obtained for N2+, NO+, and CO+. The agreement between the model calculations of ion density and the measurements is good for some species, such as O+, and rather poor for others, such as CO+, indicating that while a basic understanding of the major chemical and physical processes controlling the composition and vertical distribution of the dayside Venus ionosphere, well below the ionopause, has been achieved, there are many important details requiring further investigations.