The reactivity of each lanthanum and manganese nitrate and chloride is studied in four molten media, NaN03, NaN02, KN03 and KN02 at 500°C. The lanthanum salts produce mainly hydroxide and oxychloride. The manganese salts give a variety of compounds, mainly oxides and also oxychloride and sodium manganate. The reaction of the mixed metal salts leads to the perovskite non-stoichiometric La1-xMn03. The results can be interpreted in terms of acido-basicity of the lanthanum ion and of oxidation pathway of the manganese species. The simultaneous presence of both metal ions in the media forces Lal-xMn03 to precipitate. The obtained crystallites are nanosized with a large surface area.