We present the synthesis, structure determination, and thermodynamic properties of a never reported cocrystal prepared with lidocaine and L-menthol. The temperature-composition phase diagram of the lidocaine/L-menthol binary system was achieved using differential scanning calorimetry and X-ray diffraction experiments. The present study demonstrates that the only way to perform a phase equilibrium survey of the lidocaine/L-menthol system is to prepare the binary mixtures from the cocrystal, an equimolar stoichiometric compound of L-menthol and lidocaine. We describe a process that is crucial to elaborate pharmaceutical agents that remain in their thermodynamical stable state throughout their preparation, manufacture, and storage for effective use.