Pairs of dislocation-free molybdenum trioxide smoke crystals have been examined by transmission electron microscopy. Crystals joined on their b-faces tend to be either parallel or to have the c-axis of one perpendicular to the c-axis of the other. The preference for perpendicularity is evidence that the near-coincidence boundary that exists between perpendicular crystals has low energy. In addition to a preference for the perpendicular orientation there was a small but significant preference for the orientation obtained by rotating one member of a pair of perpendicular crystals about the normal to the contact plane until the (101) planes of one crystal are precisely parallel to the (101̄) planes of the other. The rotation (in radians) needed to do this is equal to the misfit between the a and c axes of MoO3. © 1976.