The enthalpy release during the heating of nanocrystalline platinum samples has been measured and at least two different contributions to the excess Gibbs free energy have been identified by controlling the grain size. About one half of the excess enthalpy is released during a first relaxation process without any measurable variation of grain size followed by a second relaxation which can be attributed to grain growth. The isothermal heat release of the grain growth is analyzed to calculate the excess grain boundary enthalpy.