Grain boundary (GB) segregation has been measured in Ca-doped MgO by examining intergranular fracture surfaces with Auger electron spectroscopy. The measurements reveal several interesting features. The composition of any given GB on the fracture surface is almost uniform, except for small variations due to deviations from planarity. There is a strong anisotropy of GB composition, which can amount to as much as a factor of six between low and high segregation GB's. Finally, although the compositions of opposite sides of a GB fracture are uniform, there are sometimes significant differences between the two sides, in agreement with a recently formulated model of GB composition as a function of GB character.