The half-chair conformation of the dynamically disordered molecular ring of 2-methyl-1,3-cyclohexanedione, C_7H_{10}O_2, transforms to a sofa below T_c = 244 K, when the crystal undergoes a continuous phase transition induced by the onset of halting large-amplitude vibrations of methylene groups C(4)H_2 and C(5)H_2. The temperature dependence of the crystal structure has been investigated by X-ray diffraction. The Ibam symmetry of the crystal reduces below T_c to space group Pccn. The mechanism of the phase transition and of the conversion of the ring conformation is discussed.
