Synchrotron and Guinier x-ray powder diffraction data as a function of temperature on a superconducting PbMo6S8 sample (Tc=14.7 K) showed evidence for a subtle structural phase transition at Tl=140 K, leading from a rhombohedral high-temperature to a triclinic low-temperature modification. The transition is incomplete and presents a hysteresis of about 110 K. The triclinic cell parameters obtained at 130 K, a=6.534(9) Å, b=6.532(9) Å, c=6.529(9) Å, α=89.27(8)°, β=89.12(8)°, γ=88.97(8)°, show that the distortion affects mainly the angles (δα/αr=0.3%) and less the edges (δa/ar<0.1%). This result contrasts with previous neutron diffraction work [Phys. Rev. 35, 5365 (1987)] where the distortion was found to be much greater and to affect both the angles and the edges (δa/ar=1.2%, δα/αr=1.2%), and to be of the same order of magnitude than in the MMo6S8 (M=Sr,Ba,Eu) analogues (M=Ba: δa/ar=1.2%,δα/αr=3.5%). Guinier data on an oxygen containing sample having a lower superconducting transition temperature (Tc=10.5 K) showed a smaller distortion that started at lower temperature (Tl=100 K).