Initial crystallization of Pb-deficient, lanthanum modified lead zirconate titanate (PLZT) layers followed by post-crystallization phase conversion can be used to obtain high quality PLZT thin films. However, phase evolution in Pb-deficient PLZT thin films is not well understood. To characterize phase evolution in these films, we developed a new in situ, high-temperature X-ray diffraction (XRD) measurement approach for slow heating rates. The well-characterized Pb-excess PLZT composition was used for comparison and to validate the new XRD setup described herein. During crystallization of Pb-deficient thin films, a Pb-rich/La-poor perovskite phase and Pb-poor/La-rich fluorite phase were observed to form simultaneously. The fluorite phase was observed to partially transform into a secondary perovskite phase at higher temperatures. The results obtained are discussed in view of the current understanding of phase evolution in these materials. The details of the new in situ XRD technique are also presented.