The low-temperature, Zr/Ii ordered, form of zirconium titanate has been investigated using high-resolution transmission electron microscopy in order to characterize the incommensurate structure of phases with compositions ZrTiO4 to near Zr5Ti7O24. Electron diffraction reveals that compositions with Zr:Ti between 5:7 and 1:1 have incommensurate superstructures, and phases close to 1:1 are commensurate with an a-axis repeat 2× that of the disordered structure. High-resolution images show that the a-doubling in ZrTiO4 corresponds to a new structure, one that consists of two Zr-rich, distorted octahedral layers alternated with two Ti-rich octahedral layers. The incommensurate compositions are composed of blocks of the 1:1 structure intercalated with blocks of the commensurate 5:7 structure, the latter having a tripled a-repeat and a ZTTZTT sequence of cation layers. The intercalation can be described as an “interface-modulated” structure resulting from the quasi-periodic insertion of (100) faults with displacement vector ℝ= -l/3aord in the ordered 5:7 phase. Although their spacing is variable, the faults are uniformly distributed in such a way as to produce incommensurate satellite reflections.