PbS films are used as a Pb2+-ion selective sensor and IR detector. However, this material fails at an elevated temperature. In order to get insight of the origin of its failure, we performed high temperature structural studies of PbS films using high-resolution X-ray powder diffraction. At normal temperature and pressure (NTP), PbS film has an face centered cubic lattice and is in rock-salt (B 1) structure. At NTP, the band gap of this film is 0.45 eV. Our studies show that at similar to375 K, PbS film undergoes a subtle structural transition from rock-salt (B1) to zinc-blende (B3) structure. This B3-phase of PbS film is found to be metastable. In order to correlate our findings on structural transition with the conducting characteristics, we performed a four-point probe electrical resistivity measurement on the B3-phase of PbS films. In contrast to a band gap of 0.45 eV in B1-phase, our results show that the band gap in B3-phase is 0.127 eV, less by a factor of similar to3.5. Based on our present investigation, we conjecture that the temperature induced structural transition of PbS film into a metastable B3-phase having a reduced energy band gap is the origin of the failure of PbS film as an Pb2+-ion selective sensor and IR detector at high temperature. (C) 2003 Elsevier Science B.V. All rights reserved.