The　single-crystal　X-ray　diffraction　structure　analysis　of　an　excellent　non-linear　optical　material,　viz.　II-BaB2O4　or　Ba　3(B3O6)2,　has　been　carried　out　at　163　and　293　K.　The　two　sets　of　structural　data　are　compared　and　indicate　a　significant　shortening　of　the　c　axial　length　in　the　unit　cell　at　163　K,　whereas　the　a　and　b　axial　lengths　essentially　do　not　change.　©　2006　International　Union　of　Crystallography.