Using the poke-flow technique, we determined the viscosity of SOA material generated from catechol + OH, catechol + O3, syringol + OH, and guaiacol + OH (Fig. ). These viscosities were converted from the measured exp,flow values (Fig. ) using fluid dynamics simulations. For each 205 type of BB-SOA, the viscosity increased as the RH decreased, which is expected since water acts as a plasticizer when mixed with highly viscous material. 75 Viscosity measurements at similar RH values were combined to determine overall upper and lower limits (Fig. ). Across all BB-SOA types, the viscosity increased by at least 4-5 orders of magnitude as the RH decreased from ~40 (or 50% for catechol + O3) to ≲ 3%. At RH ≳ 40%, the viscosity was < 3 × 10 3 Pa s, while at RH ≲ 3% the viscosity was > 2 × 10 ) for primary organic aerosol from biomass burning. The purple and orange bands are drawn to help guide the eye to the difference in viscosity between phenolic BB-SOA and BB-POA. For comparison, the viscosity of peanut butter is ~10 3 Pa s and the viscosity of tar pitch is ~10 Pa s. The viscosity of catechol + O3, catechol + OH, and syringol + OH were the same within the uncertainty of our measurements. For guaiacol + OH, the viscosity appears to be lower than the other three SOA types, at least at RH ~ 25%. To attempt to explain this difference, we determined the O/C ratio of each BB-SOA using an AMS, and the results are listed in Table . Viscosity is expected to depend on the O/C ratio, molecular weight, and types of functional groups of the SOA components. Table . Elemental oxygen-to-carbon (O/C) ratio for phenolic BB-SOA from AMS data. The associated uncertainty in the calculated average O/C ratio is 12% (average absolute value of relative error), based on a comparison to previously reported standards.