Atm. concentrations of 11 organophosphate esters (OPE) were measured in an urban area of Izmir, Turkey, to assess their phase partitioning. OPE octanol-air partition coefficients (KOA) and vapor pressures (PL) were measured as a function of temperature Average winter and summer Σ11OPE gas-phase concentrations were 1.77 ± 0.84 and 4.00 ± 1.77 ng/m3; particle-phase concentrations were 1.95 ± 0.77 and 1.15 ± 0.36 ng/m3, resp. TCiPP1 dominated Σ11OPE, followed by TnBP and TEP. OPE concentrations generally increased and shifted to gas-phase in summer, probably due to higher temperatures which favored partitioning to the gas-phase. Distribution between two phases covered a wide range from primarily gas-phase (TEP, TnBP) or particle-phase (EHDPP, TEHP, T2iPPP). Phase partitioning was also examined by four widely used models (KOA, Soot, Steady-State, pp-LFER). All models underestimated the majority of particle-gas partition coefficients (KP), particularly for compounds with higher volatility. Estimations based on the recently reported org matter mol. weight in urban aerosols (MWOM) and OPE activity coefficients in octanol (ξOCT) determined in this work suggested the basic assumptions of KOA-based models (i.e., ξOCT/ξOM and MWOCT/MWOM = 1) are not valid.