In order to optimise the phase equilibria of higher order alloys containing aluminium, manganese, silicon and iron, it is necessary to use thermodynamic data. In this paper, the heat of dissolution at infinite dilution of manganese in aluminium is determined, and the enthalpy of formation at 298 K of a ternary alloy alpha-AlMnSi is calculated from calorimetric measurements. These values have been measured with a Tian-Calvet calorimeter (Setaram), by dissolution in an aluminium bath at 1073 K.