We use incoherent scatter measurements of ion and electron temperature and electron density to infer exospheric neutral temperatures and thermospheric atomic oxygen densities from the ion energy balance equation. Measurements obtained at four periods in 1985 and 1986, during the recent solar cycle minimum, are examined. At all seasons, the MSIS-86 empirical model of the thermosphere is found to overestimate daytime exospheric temperatures at Arecibo by an average of about 50 ¿K. We find that daytime concentrations of H+ are not negligible at Arecibo at solar cycle minimum. Unless the presence of light ions is accounted for when analyzing autocorrelation functions measured by the incoherent scatter technique, significant errors can be made in determining the ion temperature and inferring the exospheric temperature. Deduced atomic oxygen densities are in good agreement with the MSIS model if it is adjusted to match the observed neutral temperatures and the commonly used value of the O+-O collision cross section is increased by 70%. ¿ American Geophysical Union 1988