We measured, for the first time, the oxidation enthalpies of both molybdenum (Mo) and tungsten (W) nanoclusters at several cluster sizes. We observed that the oxidation enthalpies of both Mo and W nanoclusters went down to the corresponding bulk values, respectively, with increasing cluster size. By using the oxidation enthalpies, we estimated the formation enthalpies by using H-nanocluster = O-nanocluster - O-bulk. The formation enthalpies of both Mo and W nanoclusters were all positive and went down to the bulk value (defined as 0 kcal/mol), indicating that the thermal stabilities of both Mo and W nanoclusters improved toward the bulk state with increasing cluster size. (C) 2002 Elsevier Science B.V. All rights reserved.