Thin film bicrystals of gold were annealed to create a large variety of grain boundaries of controlled crystal misorientation. The faceting of these boundaries has been studied and some low energy boundary planes identified. Tilt boundaries with a 〈110〉 misorientation axis have been found to facet more readily than those with a 〈100〉 axis. The results have been considered in terms of O-lattices and coincident site lattices but neither of these approaches is able to explain all the observations. Consquently no simple and general criteria, based on geometrical considerations, have been found for the occurrence of grain boundary planes of low energy. A further set of observations relates to the dissociation of some specific high angle grain boundaries into a twin and another low energy boundary. Boundaries with a σ33 coincidence site lattice relationship frequently appear to be characterized by a relatively low energy. © 1978.