An in situ observation method to understand thermal Marangoni convection in sessile droplets is reported in this paper. Infrared (IR) thermal imaging is applied in the experiments to visualize the transient temperature distribution on the surface of the droplet. Additionally, micro particle image velocimetry (micro-PIV) system illustrates the fluid field inside the droplet. Two kinds of droplets are considered in this paper: sessile droplets heated with a hotplate and droplets heated by an electrode at the apex. Convection in both droplets is observed in situ, and the experimental results are in accord with the theory of fluid mechanics. (C) Koninklijke Brill NV, Leiden, 2011