The evaluation of activation energies of solid-state reactions from differential enthalpic analysis (DEA) is critically assessed in the light of recent work. It is shown that the equation of Borchardt and Daniels, valid for reactions occurring homogeneously in the liquid phase, may, only under certain circumstances, be employed in one of two possible simplified forms for solid-state thermal decompositions. Results obtained by DEA and vacuum microgravimetry in the dehydration of Mn(HCOO)2,2H2O indicate that a meaningful value of the activation energy of the interfacial reaction for this solid may be obtained.