We have investigated in situ an orthorhombic (o)-tetragonal (t) phase transition of double perovskite-structured La-0.68 (Ti-0.95, Al-0.05)O-3 by high-resolution (Deltad/d approximate to 0.0002) synchrotron powder diffraction technique. The unit-cell parameters increased continuously with temperature also near the o-t transition point. Good agreement was obtained in the unit-cell parameter values between heating and cooling. These results indicate that the o-t phase transition occurs reversibly and continuously at T-c = 350 +/-2degreesC. We demonstrate that the present synchrotron X-ray powder diffraction technique is very powerful to determine both the transition temperature of a continuous transformation and precise unit-cell parameters even near the continuous transition point. (C) 2002 Elsevier Science B.V. All rights reserved.