Field studies of the viscosity and phase behavior of BBOA have relied mostly on electron microscopy measurements. In these studies, a type of BBOA known as tar balls has been identified alongside other organic particles. Tar balls are operationally defined as particles that are spherical and stable when viewed with an electron microscope. The fact that they are still spherical after collection on an electron microscope substrate indicates that the viscosity of tar balls is very high (≳ 10 9 Pa s) at the conditions used for collection. Most often electron microscopy measurements are carried out under a vacuum. Hence, a limitation of electron microscopy analysis of BBOA is that high volatility organic aerosol is likely lost in these studies. In the following, we carried out complementary studies to those described above, aiming to improve our understanding of the phase behavior and viscosity of BBOA. We investigated the viscosity and phase-behavior of BBOA samples collected from forest fire smoke in British Columbia, Canada. Viscosity was probed with the poke-flow technique and phase-behavior was measured with optical microscopy as a function of relative humidity (RH). All experiments were carried out at atmospheric pressure.