Grain-boundary (GB) segregation has been measured in Nb-doped TiO2 (rutile) by Auger electron spectroscopy analyses of the two surfaces exposed by inter-granular fracture. In addition, the orientations of the exposed GB planes on both sides of the fracture surface have been determined. A strong anisotropy of GB composition is found. Measurements on 166 individual GB fracture surfaces show that boundary planes with low and high levels of segregation tend to be clustered along the (110)-(010) and (001)-(011) regions of the stereographic triangle, respectively. The matching halves of GB planes with either high or low segregation also tend to be terminated by planes in the same respective regions of orientation space. High and low levels of segregation occur in regions of GB plane orientation space estimated to correspond to high and low GB energy, respectively. This trend is consistent with qualitative arguments based on Gibbsian adsorption.