6,6-dimethylbicyclo[3.1.1]heptan-2-one, (1R)-(+)-nopinone, is a liquid at room temperature. It solidifies below 260 K to form an orientationally disordered body-centred cubic phase (a = 7.644 Å at 255 K), which transforms to an ordered structure below 240 K. The low-temperature structure has been solved from powder X-ray diffraction data, and is orthorhombic, space group P212121, Z' = 1, Z = 4, with a = 17.6441(8) Å, b = 6.9682(3) Å, c = 6.5272(3) Å at 120 K.