The partitioning of semi-volatile compounds between the gas and particle phase influences the mass, size and chemical composition of the secondary organic aerosols (SOA) formed. Here we investigate the partitioning of water-soluble organic carbon (WSOC) and the formation of SOA in Los Angeles (LA), California and Atlanta, Georgia; urban regions where anthropogenic volatile organic compound (VOC) emissions are dominated by vehicles, but are contrasted by an additional large source of biogenic VOCs exclusive to Atlanta. In Atlanta, evidence for WSOC partitioning to aerosol water is observed throughout the day, but is most prevalent in the morning. During drier periods (RH < 70%), the WSOC partitioning coefficient (Fp) was in proportion to the organic mass, suggesting that both particle water and organic aerosol (OA) can serve as an absorbing phase. In contrast, despite the higher average RH, in LA the aerosol water was not an important absorbing phase, instead, Fp was correlated with OA mass. Particle water concentrations from thermodynamic predictions based on measured inorganic aerosol components do not indicate significant differences in aerosol hygroscopicity. The observed different WSOC partitioning behaviors may be attributed to the contrasting VOC mixture between the two cities. In addition, different OA composition may also play a role, as Atlanta OA is expected to have a substantially more aged regional character. These results are consistent with our companion studies that find similar partitioning differences for formic acid and additional contrasts in SOA optical properties. The findings provide direct evidence for SOA formation through an equilibrium partitioning process.