We show here that the densities of mass 28-ions measured by the Pioneer Venus Orbiter ion mass spectrometer (OIMS) on the nightside of Venus are highly variable and show little correlation with the values of the O+ densities. We have determined the total production rates of mass-28 ions in the chemical equilibrium region and find that this production rate cannot be explained by known chemical production reactions. We propose that the ''excess'' production is due to precipitation of electrons into the nightside thermosphere. ¿American Geophysical Union 1990