We have investigated in situ a continuous transition between the orthorhombic and tetragonal phases in perovskite-structured La-0.63(Ti-0.92,Nb-0.08)O-3 by three X-ray powder diffractometers with different Deltad/d resolutions of 0.03%, 0.06% and 0.10%. The d and Deltad denote the lambda/ (2 . sin theta) and peak width where lambda and theta are wavelength of X-ray and Bragg angle. Only the highest-resolution diffractometer of Deltad/d = 0.03% was able to detect the peak splitting between 400 and 040 reflections in the temperature range of 327-339degreesC. It was found that the accuracy of the transition temperature is considerably improved with decreasing of Deltad/d value. The maximum temperature where the peak splitting between 400 and 040 reflections is detectable increases, while the transition temperature determined by a power law decreases with decreasing of Deltad/d value. (C) 2003 Elsevier Science B.V. All rights reserved.