We have prepared and studied a new form of dehydrated magnesium formate. This form is orthorhomic ( a = 8.710(2), Å, b = 8.427(2), Å, c = 7.477(2)Å, Z = 4, Pbca). The structure ( R = 0.060, Rw = 0.049) shows that the Mg are bonded together by formate bridges, thus forming sheets parallel to the x0 y plane. Tridimensional cohesion is secured by water molecules inserted between sheets; these water molecules are bonded to the Mg of one sheet and, by hydrogen bonds, to two formate groups of the next neighbor sheet. From the structural comparison of the two forms (monoclinic and orthorhomic), a lower stability of the orthorhombic variety was predicted. This was confirmed by study of the thermal decomposition under argon atmosphere as observed by thermogravimetric and thermodifferential analysis, powder X-ray diffraction and infrared spectra.