The ZrO2-TiO2 phase diagram was determined experimentally between 800 and 1200 degrees C, 1 atm, extending our knowledge of this system to temperatures previously inaccessible for equilibrium experiments due to sluggish kinetics. The crystallization of the ordered (Zr,Ti)(2)O-4 phase from the oxides was facilitated by the addition of flux (CuO or Li2MoO4/MoO3), and seeds. Two ordered (Zr,Ti)(2)O-4 phases with different compositions were identified, and their phase relationships with TiO2 and ZrO2 solid solutions investigated. Structure data, superstructure reflections and composition were used to locate the ordering phase transition of (ZrTi)(2)O-4 in equilibrium with ZrO2 and TiO2. At the onset of ordering between 1130 and 1080 degrees C, (Zr,Ti)(2)O-4 is of composition X-Ti = 0.495 +/- 0.02, and displays a dramatic change in b-dimension. At 1060 degrees C and below, the composition of (Zr,Ti)(2)O-4 is significantly more Ti-rich and dependent on temperature, ranging from X-Ti 0.576 at 1060 degrees C to 0.658 at 800 degrees C. This variability in composition of the ordered phase contrasts with previous studies that suggested the composition to be constant at either X-Ti = 0.667 [ZrTi2O6] or 0.583 [Zr5Ti7O24]. When grown at low temperatures and with lithium molybdate, the crystals of ordered (Zr,Ti)(2)O-4 are acicular to needle shape, and develop distinct square cross-sections and end facets. (c) 2005 Springer Science + Business Media, Inc.