Phenolic compounds are largely emitted from biomass burning (BB) and have significant potential to form SOA (Phc-SOA). However, the toxicological properties of Phc-SOA remain unclear. In this study, phenol and guaiacol were chosen as two representative phenolic gases in BB plumes, and the toxicological properties of their SOA generated under different photochemical ages and NOx levels were investigated. Across explored aging conditions, oxidative potentials (OP) of Phc-SOA measured by the dithiothreitol (DTT) assay were 41.3-83.9 pmol min-1 μg-1. OH-adducts of guaiacol (e.g., 2-methoxyhydroquinone) were identified as components of guaiacol SOA (GSOA) with high OP. The addition of nitro groups to 2,5-dimethyl-1,4-benzoquinone, a surrogate quinone compound in Phc-SOA, increased its OP. In pure water, H2O2 presented the main reactive oxygen species produced by Phc-SOA. The toxicity of both phenol SOA (PSOA) and GSOA in vitro in human alveolar epithelial cells decreased with aging in terms of both cell death and cellular ROS, possibly due to more ring-opening products with relatively low toxicity. The influence of NOx was consistent between cell death and cellular ROS for GSOA, but not for PSOA, indicating that cellular ROS production does not necessarily represent all processes contributing to cell death caused by PSOA.