Biomass burning, including wildfires, emits large amounts of volatile organic compounds (VOCs) into the atmosphere. One major class of VOCs emitted from biomass burning is phenolic compounds. Once in the atmosphere, phenolic compounds will be oxidized by OH, O3, and NO3 to form lower volatility products, which can form new secondary organic aerosol (SOA) particles or partition to the particle phase to grow pre-existing SOA. SOA can contribute to poor air quality and can influence climate by directly scattering or absorbing solar radiation or by acting as nuclei for clouds. To predict the environmental impacts of SOA, information on their volatility and chemical composition is needed. Thermal desorption (TD) techniques are widely used to determine both of these properties. TD techniques coupled with particle sizing instruments are often used to determine the volatility of SOA, and TD techniques coupled with mass spectrometers are often used to determine both volatility and chemical composition. To interpret TD measurements, information on the thermal stability of SOA components is needed. Several previous studies have investigated the thermal stability of SOA components. However, no studies have investigated the thermal stability of catechol + O3 SOA. Catechol (Fig. ) is a phenolic VOC emitted during biomass burning, and it produces SOA upon oxidation by O3. To interpret results from TD measurements, information is also needed on the viscosity of SOA when heated. When converting TD measurements to volatilities, researchers have often assumed that the SOA particles are well mixed, i.e., that the organic molecules are homogeneously mixed throughout the SOA particle. This assumption implies that the SOA viscosity is low and viscosity is not a kinetic barrier to evaporation. Alternatively, researchers have assumed that viscosity can provide a kinetic barrier to evaporation, but this kinetic barrier does not change with temperature and can be described using an effective mass accommodation coefficient. Information on the viscosities of SOA at temperatures commonly used in TD measurements is needed to test these assumptions. Many studies have investigated the viscosities of SOA at room temperature, but less attention has focused on the viscosity of SOA upon heating, and no studies have investigated the viscosity of phenolic SOA upon heating.