AbstractIn situ and operando experiments play a crucial role in understanding the mechanisms behind catalytic processes. In these experiments it is important to have precise control over pressure and temperature. In this work, we use luminescence thermometry to map the temperature distribution in a 300 μm microelectromechanical system nano‐reactor with a resolution of ca. 10 μm. These measurements showed a temperature gradient between the center and edge of the heater of ca. 200 °C (at Tset=600 °C) in vacuum and, in addition, a large offset of the local temperature of ca. 100 °C (at Tset=600 °C) in a non‐vacuum (i. e., air, He and H2) environment. The observed temperature heterogeneities can explain differences observed in the reduction behavior of Co‐based Fischer‐Tropsch synthesis catalyst particles at different locations in the nano‐reactor as determined by scanning transmission X‐ray microscopy.