A theoretical study on the combination of a passive microwave and a GNSS radio occultation instrument, mounted on the International Space Station (ISS), was performed. Both instruments allow to deduce the atmospheric temperature profile. We combined realistically simulated data of the two highly synergistic sensors with the optimal estimation method (OEM) and retrieved single joint temperature profiles from 0 to 90 km altitude for which we found accuracies <1 K below 35 km and <4 K in the mesosphere, respectively. In addition, we performed simultaneous tropospheric water vapor and temperature retrievals leading to a water vapor accuracy <10 to 20% at altitudes below 5 to 8 km. The OEM allows to optimally exploit the synergy in the data and maximizes retrieval accuracy over the full range from surface to mesopause. It seems worthwhile to install this type of instrument combination on the ISS or other low-Earth-orbit (LEO) platforms. ¿ 2001 American Geophysical Union