AbstractCyclododecane crystallises at room temperature in the monoclinic system,a= 13.27 Å,b= 8.28 Å,c= 5.44 Å, β = 99° 32′ (Z = 2), space group C2/m. The crystal structure was determined from the three‐dimensional PATTERSONfunction and refind by three‐dimensional difference syntheses. Because of the large temperature factor and of disorder arising from the presence of statistical mirror planes aty= 0 andy= ½, the observed data are almost equally compatible with two possible molecular conformers. Both of these are built of four nearly planar zig‐zag units each of four atoms, with an atom shared in common between successive units, but the conformations about the CC bonds formed by the common atoms are all anti‐skew for one model and all syn‐skew for the other. Reasons are given for regarding the syn‐skew mod