The problem of solute segregation came to prominence with relation to studies of the temper embrittlement of low alloy steels. McLean and Northcott1(1948) first suggested that segregation of various elements to the grain boundaries was primarily responsible for the intergranular fracture observed in steels susceptible to this embrittlement. Direct evidence for segregation came much later with the development of surface sensitive analytical techniques, especially Auger electron spectroscopy (AES). Using AES, it was determined that impurity elements such as P, Sb and Sn, as well as alloying elements such as Ni and Cr, were highly concentrated at the fracture surfaces in embrittled steels2. It is not clear, however, why solute segregation changes the mechanical strength of the grain boundaries in these materials. Based on recent calculations, Messmer and Briant3 proposed that certain solute species at a grain boundary change the chemical bonding at the interface. However, other more dramatic structural rearrangements may be possible upon segregation. Such structural changes were first suggested by the observation of facetted fracture surfaces in tellurium-doped iron alloys4. In the study presented here, it is shown that low concentrations of solute can cause changes in grain boundary structure. In particular, small concentrations of Au solute were found to cause a major change in the dislocation structure of low angle [001] Fe twist boundaries. Preliminary observations on the str ucture of a Fe-0.18 at.% Au* twist boundary were presented elsewhere5. Additional results will be presented here on the effect of changes in solute concentration and misorientation angle, θ, on this structural transformation. It is believed that these observations are evidence for the occurrence of a two-dimensional phase transformation in the grain boundary, similar to that predicted by Hart6