Organic-inorganic hybrid materials have enormous potential for applications in catalysis, gas storage, sensors, drug delivery, and energy generation, among others. A class of hybrid materials adopts the ABX(3) perovskite topology. We report here the synthesis and characterization of an isostructural series of dense hybrid perovskites, [(CH3)(2)NH2] [M-(HCOO)(3)], with M = Mn, Co, Ni, and Zn. These compounds have shown promising multiferroic behavior. Understanding their stability is crucial for their practical application. We report their formation enthalpies based on direct measurement by room-temperature acid solution calorimetry. The enthalpy of formation of this dimethylammonium metal formate series becomes less exothermic in the order Mn, Zn, Co, Ni. The stability of the hybrid perovskite decreases as the tolerance factor increases, unlike trends seen in inorganic perovskites. However, the trends are similar to those seen in a number of ternary transition metal oxides, suggesting that specific bonding interactions rather than geometric factors dominate the energetics.