The phase transition occurring at Tt = 528 K in the natural mineral and synthetic laser host compound KPb2Cl5 has been characterized by means of both single-crystal X-ray diffraction (XRD) as a function of temperature and differential scanning calorimetry (DSC). The K+ and Pb2+(2) cations order on passing from the high-temperature orthorhombic phase (space group Pmcn; a = 8.951, b = 8.015, c = 12.683 Å, Z = 4 at 623 K) to the low-temperature monoclinic one (space group P21/c; a = 8.849, b = 7.918, c = 12.472 Å, = 90.11°, Z = 4, at room temperature), leading to a group-subgroup first-order phase transition displaying moderately fast kinetics and low entropy production upon thermal cycling around Tt. The positional entropy calculated from our crystal structure models corresponds to the transition entropy measured by DSC within the experimental error, and amounts to about 0.34 R. The driving force for the phase transition, A 22 J mol-1, remains negligible with respect to both the thermal energy at Tt (RTt 4.4 kJ mol-1) and the transition latent heat (H 790 J mol-1), thereby suggesting that this phase transition is a close-to-equilibrium process.