Here, we focus on the unique physical and chemical properties of catechol + O3 SOA when heated to mild temperatures (36-52 °C). We used an optical microscope to monitor the change in size and morphology during heating of the SOA to mild temperatures (36-52 °C). To better understand the optical microscope results and the thermal stability of the components, we used three different mass spectrometry techniques to measure the composition of the gas and particle phase before, during, and after heating. We also calculated the viscosity of the SOA at temperatures up to 52 °C. The implications of these results for interpreting TD measurements are discussed.