The three known polymorphs of malonamide have been characterized by thermal analysis and X-ray powder diffraction. The melting thermodynamic characteristics of the three forms are thus proposed in the present paper. From these data, the relative thermodynamic stability of these three solid forms has been determined. It appears that an enantiotropic behavior is established between the monoclinic and the orthorhombic phases while the quadratic one is monotropic with respect to the other two.