In this work the solid–liquid equilibrium (SLE) and freezing-point depression (FPD) in the electrolytic binary aqueous systems piperazine (PZ, CAS No. 110-85-0) and aqueous 2-amino-2-methyl-1-propanol (AMP, CAS No. 124-68-5) were measured. The FPD and solubility were also determined in the ternary AMP–PZ–H₂O system. A method was developed by which solubility can be determined at higher temperatures using the FPD setup. A total of 86 data points are listed in the full concentration range from (−35 to 90) °C. The solid phases piperazine hexahydrate (PZ·6H₂O), piperazine hemihydrate (PZ·1/2H₂O), and anhydrous PZ precipitated during the experiments. The data can be used in the formulation, prevention, or intentional formation of slurries in piperazine solvents for promoting CO₂ capture using absorption and desorption.