Experimental methods used to determine liquid-liquid equilibria (LLE) in systems containing ionic liquids (ILs) are summarized. While these methods are essentially the same as in studies of LLE in systems of molecular liquids, systems containing ILs show some specificities that must be taken into account. The methods addressed are therefore compared and assessed as to their suitability, advantages and drawbacks. In the second part of the chapter, data analysis is addressed to demonstrate the wealth of information that can be obtained by means of using appropriate methods. An example of multivariate calibration using the partial least-squares method is given to show the means of separating overlapping UV-Vis spectra. Most importantly, though, a brief introduction to mathematical gnostics is given. This nonstatistical approach to data uncertainty is a powerful tool in critical assessment of a number of types of thermodynamic data, including LLE, and in a robust linear as well as nonlinear regression even of small samples of scattered data.