After being dormant for more than 9000 years, Mount Chaiten in southern Chile started to erupt in the early morning (around 0430-0600 UTC) of 2 May 2008. In this work we combine data from FORMOSAT-3/COSMIC and the NCEP GFS model to reveal spatial and temporal evolution of temperatures following the eruption. The eruption occurred when there were active synoptic waves passing over South America. As a result, the volcanic plumes were carried downstream in the westerly flow to the eastern South Atlantic over several days following the eruption. Estimated peak temperature reductions of up to 10 K associated with the volcanic plume were confined to downwind locations and at altitudes between 12 and 20 km. Temperature increases in the troposphere at altitudes below 10 km were associated with the transport of warm air from low latitudes toward midlatitudes. This work shows that FORMOSAT-3/COSMIC is useful for near real-time global temperature monitoring. Citation: Wang, K.-Y., S.-C. Lin, and L.-C. Lee (2009), Immediate impact of the Mt Chaiten eruption on atmosphere from FORMOSAT-3/COSMIC constellation, Geophys. Res. Lett., 36, L03808, doi: 10.1029/2008GL036802.