The structure and thermodynamic properties of a Σ5 (001) twist boundary in gold are studied as a function of temperature. This study was performed within the framework of the Local Harmonic (LH) model and employed an Embedded Atom Method (EAM) potential for gold. We find that for the Σ5 (001) twist boundary in gold, a distorted CSL structure is stable at low temperatures, but undergoes a phase transformation to a DSC related structure near room temperature. This transformation is shown to be first order. The temperature dependences of the excess grain boundary free energy, enthalpy, entropy, specific heat, and excess volume are calculated. Discontinuities are observed in the slope of the grain boundary excess free energy (versus temperature), in the value of the grain boundary excess specific heat and excess volume. The stable high temperature grain boundary structure has a smaller excess volume than does the lower temperature structure, and both structures have a coefficient of thermal expansion which is in excess of that for the perfect crystal.