Here, we measured the RH-dependent viscosity of SOA particles at 294 K generated by reacting NO 3 with guaiacol (2-methoxyphenol), referred to as guaiacol-NO 3 SOA. Guaiacol, a phenolic compound, is emitted from biomass burning events and can contribute significantly to SOA formation in the atmosphere. We use guaiacol as a model system for phenolic compounds from biomass burning events. We found that the viscosity of this SOA is extremely high (≳ 5×10 Pa s) at RH ≲ 70%. In addition to measuring the RH-dependent viscosity of guaiacol-NO3 SOA, we also developed a parameterization for predicting the viscosity of this SOA as a function of RH and temperature, and used this parameterization to predict the viscosity and mixing times within phenolic SOA in the atmosphere.