A new chiral 6:7 open-framework uranyl molybdate, [C 6 H 16 N] 2 [(UO 2 ) 6 (MoO 4 ) 7 (H 2 O) 2 ](H 2 O) 2 , has been synthesized by hydrothermal methods. The structure has been refined using single-crystal X-ray diffraction data collected at 20 and −127°C. The 20°C structure [orthorhombic, C222 1 , a=11.3045(14), b=19.962(6), c=24.416(5)Å, V=5510(2)Å 3 ] has been refined to R 1 =0.046 on the basis of 6093 unique observed reflections. The −127°C structure [orthorhombic, P2 1 2 1 2 1 , a=11.211(4), b=19.880(10), c=24.421(8)Å, V=5443(4)Å 3 ] has been refined to R 1 =0.047 on the basis of 6951 unique observed reflections. The structures are based upon topologically identical frameworks of corner-sharing UO 7 pentagonal bipyramids and MoO 4 tetrahedra. The extra-framework H 2 O groups and protonated triethylamine molecules reside in the framework cavities. In the C222 1 structure at 20°C, H 2 O and [C 6 H 16 N] + molecules filling the chiral channels along [001] are disordered, whereas, in the P2 1 2 1 2 1 structure at −127°C, they are perfectly ordered. The symmetry difference between structures at 20°C and −127°C is the result of a C222 1 →P2 1 2 1 2 1 second order phase transition that involves ordering of extra-framework protonated amine molecules and H 2 O groups, and distortion of the flexible [(UO 2 ) 6 (MoO 4 ) 7 (H 2 O) 2 ] 2− uranyl molybdate framework. On the basis of measurements of intensities of reflections that violate absence conditions of C-centering cell, the temperature of the phase transition is estimated as −11°C.