We measured the change in enthalpy with grain size of a dense nanograined yttria-stabilized zirconia by oxide melt solution calorimetry and derived a grain boundary enthalpy, 0.73 +/- 0.19 J m(-2). Surface enthalpies of nanopowders are 2.21 +/- 0.14 J m(-2) (anhydrous surface) and 1.60 +/- 0.09 J m(-2) (hydrous surface). The grain boundary enthalpy is about a factor of two smaller than the enthalpy of the anhydrous surface, suggesting that densification which maintains nanosized grains is indeed thermodynamically driven. This is the first direct calorimetric measurement of grain boundary enthalpy in a ceramic.