Atmospheric concentrations of 11 organophosphate esters (OPEs) were measured in an urban area in Izmir, Turkey to explore their phase partitioning. Octanol-air partition coefficients (K-OA) and vapor pressures (P-L) of the OPEs were also measured as a function of temperature. Average Sigma 11OPE gas-phase concentrations were 1.77 +/- 0.84 and 4.00 +/- 1.77 ng/m(3), while particle-phase concentrations were 1.95 +/- 0.77 and 1.15 +/- 0.36 ng/m(3) during winter and summer, respectively. TCiPP1 dominated Sigma 11OPEs, followed by TnBP and TEP. OPE concentrations generally increased and shifted to gas-phase in the summer probably due to higher temperatures that favor partitioning to the gas-phase. Distribution between two phases covered a wide range from being primarily in gas-phase (TEP, TnBP) or particle-phase (EHDPP, TEHP, T2iPPP). Phase partitioning was also examined via four widely used models (K-OA, Soot, Steady-State, and pp-LFER). All models underestimated the majority of particle-gas partition coefficients (K-P) especially for the compounds having higher volatilities. Estimations based on the recently reported molecular weight of organic matter in urban aerosols (MWOM) and activity coefficients of OPEs in octanol (xi(OCT)) determined in the present study suggested that the basic assumptions of K-OA-based models (i.e., xi(OCT)/xi(OM) and MWOCT/MWOM = 1) are not valid.