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Large volumes of borate resources exist in Bolivia, with the most important being the Rio Grande deposit, located close to the Salar of Uyuni. Here, borates occur in beds and lenses of variable thickness. A mineralogical and thermal characterization of borates from the Rio Grande was made using XRD, FTIR, SEM and DTA–TG. The deposit is mainly composed of B2O3, CaO and Na2O, with minor contents of MgO and K2O. Some outcrops are constituted by pure ulexite aggregates (NaCaB5O6(OH)6·5H2O) of fibrous morphology; in other cases, gypsum, calcite and halite also are present. The thermal decomposition of ulexite begins at 70 °C and proceeds up to ~550 °C; this decomposition is attributed to dehydration and dehydroxylation processes in three steps: at 115, 150–300 and 300–550 °C. The last mass loss of 1–5 % at 800 °C is due to the removal of Cl2 from the decomposition of halite. DTA shows two endothermic events related to the removal of water; in the first, NaCaB5O6(OH)6·5H2O evolved from NaCaB5O6(OH)6·3H2O, at 108–116 °C; in the second, NaCaB5O6(OH)6 is formed at 180–185 °C and NaCaB5O9 (amorphous) is formed at 300–550 °C. The exothermic peak (658–720 °C) is related to the crystallization of NaCaB5O9. A small endothermic peak appears due to the halite melting. Later, another endothermic event (821–877 °C) appears, which is related to the decomposition of NaCaB5O9 into a crystalline phase of CaB2O4 and amorphous NaB3O5. The XRD pattern evidences that, at 1050 °C, CaB2O4 still remains in the crystalline state.