The dayside ion concentrations in the Venus ionosphere obtained by the Pioneer Venus orbiter ion mass spectrometer exhibit a modulation corresponding to the 27-day solar variation. Comparisons were made of the amplitudes of modulation of CO2+, C+, and O2+, with the amplitudes of the 27-day variation in the 10.7 cm solar radio flux and the simultaneously measured EUV fluxes at He II (304 A) and Lyman &bgr; (1026 A), together with a theoretical analysis of the effects of solar variability on the ionosphere and neutral atmosphere of Venus. This analysis leads us to the conclusion that the observed modulation of dayside ion densities is primarily due to the variability in the ionizing EUV radiations and, to a much lesser extent, the result of the variability with solar activity of the neutral atmosphere via the variability in exospheric temperature. In this connection, we also show, theoretically, why the percentage variation of exospheric temperature on Venus (as observed in the ONMS data) for a given variation in F10.7, is only half of the exospheric temperature variation for Earth.