The minerals manganite, pyrolusite, ramsdellite, cryptomelane, psilomelane, braunite, lithiophorite and woodruffite from Sandur manganese ore deposits, Bellary District, Mysore State, which have been investigated by microscopic, chemical and X-ray methods (Naganna, 1963) are subjected to thermal study through differential thermal analysis and gravimetric thermal analysis. The hydrous oxides of manganese undergo dehydration below 400° C. and subsequent changes follow later. Most of the minerals containing MnO₂ thermally react in two stages, one at about 600° C. and the other beyond 800° C. The thermal behaviour of pyrolusite is seen to be similar to that ofβ MnO₂, while the thermal behaviour of ramsdellite is found to be similar to those ofα orγ MnO₂. Further the study suggests that ramsdellite may change over to pyrolusite below 500° C.