In this work, the solid–liquid equilibrium (SLE) of the ternary system 2-amino-2-methyl-propanol (AMP)–piperazine (PZ)–H2O and the aqueous binary system AMP was determined using a freezing point depression setup and differential scanning calorimetry. A total of 59 new data points are listed in the full concentration ranges of 0 < w(AMP) < 100% and 0 < w(PZ) < 61%. The SLE phase boundary of AMP tetrahydrate (AMP·4H2O) was determined and confirmed by its crystal structure obtained by powder X-ray diffraction. The new data of this work can be used in the creation of a thermodynamic model for prevention of AMP and PZ precipitates from solvents used for CO2 capture. This gives a higher degree of safe operation. These data will allow for a deeper understanding of molecular interaction and bond lengths in the AMP–water system, which can be used for molecular simulation.