Equilibrium dialysis data has provided valuable information concerning the preferential interaction of a cosolvent with a biomolecule in aqueous solutions. Here, we formulate the experimental data in terms of Kirkwood-Buff (KB) theory, resulting in equations that provide a simple physical picture of the dialysis experiment and thereby the interaction of a cosolvent with a biomolecule. These results are then used to establish exact relationships between preferential interaction coefficients, defined in different ensembles and/or using different concentration scales, in terms of KB integrals. It is then argued that the molality based equilibrium dialysis data represent the situation most relevant to computer simulations performed in either open or closed systems.