Thermo-magnetic measurements and thermal analysis confirm the structure of the binary system of manganese and indium in agreement with U. Zwicker. All alloys in the liquid phase obey the Curie- Weiss law as can be seen from the characteristic curves and can be evaluated according to the ion model. The results are compared with those of the manganese-gallium system. When the manganese atom is introduced into the molten B-metal it becomes bivalent in all three binary systems.