The -65 °C phase transition in adamantane, C10H16, involves a change from the face centered cubic room-temperature structure to a tetragonal structure, space group PTt21c, with a=6-60, c=8-81 /~ and Z=2. The low-temperature cell is related to the cubic cell by atetr~-acubic/)/2 and ctetr-~aeubic. A single-crystal X-ray study of the low-temperature phase at -110 °C shows the arrangement of the molecules to be unchanged except for a 9 ° tilt about the c axis. A redetermination of the room-temperature structure using single-crystal data supports a disordered structure, space group Fm3m, rather than the previously assumed, ordered, F]3m structure.