X-Ray crystallographic studies on a 3 : 1 thiourea–ferrocene clathrate have been carried out at 295 and 100 K. The crystal structure of the high-temperature form has been determined from diffractometer data by Patterson and Fourier methods. Crystals are rhombohedral, space group Rc with Z= 6 in a unit cell based on the hexagonal setting of a= 16.360(3) and c= 12.395(2)Å. The structure has been refined by least-squares methods to R 0.079 for 711 observed reflections. It consists of thiourea molecules which form a honeycomb of channels by spiralling with a pitch of 120 ° parallel to the c axis. Within these channels, sites of point symmetry 32 are occupied by the ferrocene iron atoms. The cyclopentadienyl rings are disordered and the time-averaged picture shows regions of three-dimensionally delocalised cyclopentadienyl electron density around the iron atoms.