The structure and energy of two 36.9° grain boundaries, one between (100) planes and the other between (110) planes, were calculated for gold and three gold-copper alloys. Three types of alloys were examined : (a) an ordered alloy containing an equal number of gold and copper atoms, (b) a gold rich alloy in which only one copper impurity atom was associated With each periodic unit of boundary structure, and (c) a two phase alloy with an interphase boundary between a pure gold grain and a pure copper grain. In this alloy system, which is characterized by a high degree of solubility, the grain boundary structures were not found to be sensitive to the alloying conditions ; however, there was a tendency in dilute solution for foreign atoms to segregate to the grain boundary. Several of the existing methods of describing the grain boundary structure are discussed and a new alternative method of describing the structure is proposed