An orientationally disordered crystalline (ODIC) plastic phase (γ) was observed in Trimethylolpropane (TRMP) during heating by high resolution thermal and X-ray diffraction analyses. TRMP is a potential thermal energy storage material. The enthalpies of solid-solid (α → γ at 327.8 K) and fusion (γ → liquid at 332.7 K) transitions are 16.36 kJ/mol and 0.9 kJ/mol, respectively. Supercooling was observed during solidification of melts, and this supercooled γ phase began to transform to a metastable crystalline phase, designated as α′, after 20 minutes at room temperature. The lattice parameters of the monoclinic α phase, obtained from this study, are: a = 0.8427(5), b = 0.9580(5), c = 0.9185(6) nm, β = 98.958(3)° @ 298 K with Z = 4. The ODIC high temperature FCC γ phase has a lattice parameter: a = 0.9274(3) nm @ 328 K with Z = 4. Differential scanning calorimetry, in-situ Bragg-Brentano X-ray diffractometry, and high temperature and resolution Guinier X-ray diffraction systems were used to determine these phase transitions.