Thermodynamic dependencies and information are of prime importance in performing a controlled selective crystallization process to separate, e.g., the enantiomers of chiral compounds. This study is focused on the determination of phase diagrams of a chiral system. The melting behavior and solubility equilibria (SLE) of diethyl-1-(2,4-dichlorophenyl)-5-methyl-4,5,dihydro-1H-pyrazole-3,5-dicarboxylate (mefenpyr-diethyl) in various ethanol/water mixtures in the temperature range of (273 to 293) K were analyzed. The obtained results identify mefenpyr-diethyl as a racemic compound-forming system. It has been found that the solubility behavior strongly depends on temperature. In certain concentration ranges, liquid liquid equilibria (LLE) were detected: Liquid-liquid separation has to be prevented during cooling crystallization processes. The observed LLE phenomena were quantified within this study in polythermal (turbidity) measurements, and the phase diagrams were derived on the basis of the experimental data. With these diagrams, a successful selective crystallization process avoiding oiling out is feasible.