[en] The structure of CeCu₆ has been studied at eight temperatures from 10K to 295K by time-of-flight neutron powder diffraction. The high temperature orthorhombic cell transforms to a monoclinic cell at 230K. Rietveld structural refinements at each temperature give details of the structural distortion associated with the transition. The space groups and the temperature dependence of the monoclinic strain are consistent with a second order transition driven by a soft acoustic phonon mode. However, small atom displacements perpendicular to the shear direction suggest that an optic mode may also be involved in the transition