Solute atom segregation and phase transitions at internal interfaces in metal alloys are studied using sophisticated high resolution techniques. The objective is to understand the segregation of solute atoms as a function of the structure of an interface and the thermodynamics of the bulk system. The chemical compositions of individual boundaries are measured by atom probe field ion microscopy and atomic resolution analytical electron microscopy. Simulation techniques are employed to explore the eight-dimensional hyperspace of a grain boundary with respect to solute atom segregation and phase transitions. Both tilt and twist grain boundaries are studied with different crystallographic interfaces. %%% Interface properties play a major role in determining important properties of metal alloys that can determine their end use, such as a structural material or magnetic material. This research should result in a better understanding of solute segregation at interfaces, which in turn affects the interfacial properties.