The measured diffusion coefficients of R6G through the two BBOA phases measured using rFRAP as a function of RH at 293 K are reported in Fig. (left axis), with the derived viscosity reported on the right axis. Throughout the entire RH range studied, the hydrophobic phase exhibits a constant viscosity of 10 2 Pa s within the degree of experimental uncertainty. This is roughly the viscosity of ketchup. On the other hand, the viscosity of the hydrophilic phase is strongly dependent on the RH, decreasing from 10 3 Pa s (similar to peanut butter) to less than 10 0 Pa s (that of clear honey) between RHs of 10% and 75%. The decreasing viscosity with increasing RH of the hydrophilic phase is expected since increasing RH leads to water uptake and a corresponding decrease in the viscosity of hydrophilic and highly viscous particles and films, a process known as plasticization . To parameterize the viscosity of the two BBOA phases as a function of RH, we fit our data to an Arrhenius mixing rule (Section S2). This expression has been used in the past to successfully describe the RH-dependent viscosity of a range of aerosol compositions . The Arrhenius fit to the data (lines in Fig. ) shows that at 293 K and high RH conditions the viscosity of the hydrophobic phase can be up to five orders of magnitude greater than the hydrophilic phase.