An experimental binary phase diagram of 2-Amino-2-methyl-1, 3-propanediol (AMPL)-tris(hydroxymethyl)aminomethane (TRIS) is developed by using high temperature in-situ X-ray diffraction (XRD), differential scanning calorimetry (DSC), and thermodynamics modeling. The organic thermal energy storage materials, such as AMPL and TRIS, undergo a solid-solid state phase transition storing large amounts of thermal energy in O-H...O and O-H...N bond rotation. High temperature in-situ XRD and DSC experiments are used to study binary equilibrium AMPL-TRIS phase diagram. Binary solid solutions of AMPL and TRIS exhibit an orientationally disordered BCC structure, in which thermal energy is stored. A computer developed AMPL-TRIS phase diagram by CALPHAD methodology agrees reasonably well with the experimental one. Gibbs energies of pure components were described based on their heat capacity data. The Gibbs energies of metastable phase modifications were establisded by assuming ideal solutions of the phases. The excess Gibbs energy parameters were optimized using Parrot program of Thermo-Calc software to include the experimental data. The phase diagram consists of 5 single phase regions, low temperature α AMPL-rich Monoclinc phase (P21/n), β-TRIS rich orthorhombic phase, (Pn21a), γ-AMPL rich and γ' -TRIS rich, both BCC orientationally disordered phases and Liquid phase. There are six two phase regions: (α+β), (β+γ), (β+γ'), (γ+γ'), (L+γ), and (L+γ'). A low temperature eutectoid transformation is observed with invariant point 348 K and 12%TRIS. As the temperature is increased, another higher temperature eutectoid transformation is observed with invariant point 388 K and 54%TRIS. A peritectoid transformation is also observed with invariant point 401 K and 38%TRIS. © 2015 Elsevier Ltd. All rights reserved.