Surfactant mediated enhanced oil recovery techniques such as surfactant-polymer (SP), and alkalinesurfactant-polymer (ASP) flooding, have received increased attention in the past decade due to their ability to reduce capillary forces at the pore-scale to ultra-low values and mobilize oil. In order to improve the economics of such processes, a better understanding of the interaction of surfactant-oil-brine systems is needed. Recently, an equation-of-state (EoS) has been developed that significantly improves microemulsion phase behavior prediction based on coupling of the HLD-NAC equations with a new correlation of optimum conditions, and an updated hydrophilic-lipophilic difference (HLD) model that includes pressure (Ghosh and Johns 2014, 2016). The new EoS accounts for gradients in key formulation properties relevant for reservoir-scale processes in a robust and noniterative framework. One common assumption of all existing microemulsion phase behavior models is the assumption of pure excess phases, which says that the surfactant component can only be present in the microemulsion phase. The focus of this paper is to allow for surfactant partitioning into both the water and oil excess phases, and then relate the relevant surfactant partitioning coefficients to the HLD state function so that K-values, and potentially phase behavior, can be predicted. This paper derives important relations of the surfactant partition coefficient between relevant phases based on a chemical potential description of the HLD, which measures the deviation from optimum conditions. In particular, we calculate the surfactant partition coefficients from numerous experiments in the literature and show that the surfactant partitioning coefficient has a linear scaling with HLD for a wide variety of formulation variables. Surfactant K-values between excess phases from all experiments examined collapse onto the same linear relationship so that once HLD is known (based on formulation variables such as salinity) the K-values are explicitly determined. The estimation of surfactant partition coefficients can be used to aid the design of surfactant formulations for EOR processes.