Methods comprising: providing a test substance, providing two solvents that are substantially immiscible, introducing a known amount of the test substance and known amounts of the two solvents into a single vessel to create a pre-equilibrium sample, adjusting the concentration of the test substance in the pre-equilibrium sample so that the concentration of the test substance is below the critical micelle concentration of both solvents if the concentration of the test substance is not already below the critical micelle concentration, allowing the test substance to equilibrate between the two solvents over time at a substantially constant temperature, determining the equilibrium concentration of the test substance in each of the solvents, and calculating the partition coefficient.