Abstract. The volcanic cloud top altitude and the atmospheric thermal structure after volcanic eruptions are studied using Global Positioning System (GPS) Radio Occultation (RO) profiles co-located with independent radiometric measurements of ash and SO₂ clouds. We use the GPS RO data to detect volcanic clouds and to analyze their impact on climate in terms of temperature changes. We selected about 1300 GPS RO profiles co-located with two representative eruptions (Puyehue 2011, Nabro 2011) and found that an anomaly technique recently developed for detecting cloud tops of convective systems can also be applied to volcanic clouds. Analyzing the atmospheric thermal structure after the eruptions, we found clear cooling signatures of volcanic cloud tops in the upper troposphere for the Puyehue case. The impact of Nabro lasted for several months, suggesting that the cloud reached the stratosphere, where a significant warming occurred. The results are encouraging for future routine use of RO data for monitoring volcanic clouds.