Electron diffraction and imaging techniques have been used to examine the structure and thickness of low-angle grain boundaries in olivine. For edge-on grain boundaries, a periodic array of dislocations in a boundary gives rise to sets of periodic rods of diffracted intensity that are recorded as streaks in the diffraction pattern. The length of these streaks provides a measure of the structural width of a boundary. For olivine, the structural width of a boundary with a given dislocation spacing is three to four times smaller than that in metals where the boundary width is approximately equal to the spacing between dislocations in the boundary.