Thermodynamic factors, which convert species concentration gradients (∇ci) into electrochemical-potential gradients (∇μi), are essential quantities within concentrated-solution transport theory. Originally suggested by Darken [1], thermodynamic factors (χij) are defined through thermodynamic derivatives, asRT χij = yi ∂μi/∂yj,in which yi is the particle fraction of species i, R the universal gas constant, and T the absolute temperature. Accurate parameterisation of thermodynamic factors is important for the practical modelling of electrochemical systems, because they quantify how concentration polarization translates into concentration overpotential.The most common electrochemical approach to thermodynamic-factor characterisation is the concentration-cell method, whereby the steady-state voltage difference between two reservoirs containing different salt concentrations – i.e., the liquid-junction potential – is measured. A series of such measurements over a range of salt concentrations suffices to produce a functional form of χij, provided that the transference number is known from an auxiliary experiment. This method is well-established for binary electrolytes (i.e., a single salt in a neutral solvent), and has been used to produce ample data in the lithium-battery literature, for example by Nyman et al. [2], Landesfeind and Gasteiger [3], and Wang et al. [4].In practice, however, many electrochemical devices – especially lithium batteries – use electrolytes that incorporate a mixture of solvents. The relationship between voltage readings and thermodynamic factors in concentration-cell experiments becomes immediately unclear in these more complicated systems. In this talk we consider the thermodynamics of multi-solvent systems in the simplest case: that of two neutral solvents and a binary electrolyte, such as the LiPF6:EC:EMC solutions commonly used in Li-ion battery applications. Experimental procedures for characterising independent thermodynamic factors in two-solvent systems will be discussed.