AbstractThe crystal structures ofcis‐1,6‐cyclodecanediol (1) and of the molecular compound formed betweencisandtrans‐1,6‐cyclodecanediol in the ratio 2:1 (2) have been established by X‐ray diffraction. The two substances are closely isostructural. (Crystal data: both monoclinic, P21/c, Z = 6; (1)a= 6.665(6),b= 18.074(3),c =14.351(8) Å, β = 119.77(6)°,V= 1500(2) Å3; (2)a= 6.679(8),b =18.074(4),c= 14.314(10) Å, β = 119.64(8)°,V= 1502(3) Å3). The asymmetric unit of the molecular compound contains onecismolecule in a general position and onetransmolecule at a center of symmetry. In the crystal of the purecisdiol a set of disorderedcismolecules mimics the corresponding centrosymmetrictransmolecules in the molecular compound. The molecules have the stable BCB cyclodecane ring conformation, except for the disorderedcismolecules, which have a conformation about 2 kJ mol−1higher in energy (molecular mechanics calculations). The intricate three‐dimensional hydrogen‐bond network found in both crystals is presumably especially favorable from