The crystal structures of [4.4.4]propellane (monoclinic, a=12.053, b=7.832, c=13.001 Å, β=104.89°, space group C2/c, Z=4) and [4.4.4]propellatriene (monoclinic, a=7.876, b=12.651, c=13.164 Å, β=122.81°, space group P2₁/c,Z=4) have been determined by X-ray analysis. In propellane the six-membered rings are in the chair conformation, in propellatriene they adopt the ‘half-chair’ conformation with a twofold axis passing through the centre of each double bond. In both cases the observed parameters correspond to virtual D₃(32) molecular symmetry. Corrections have been applied for the effect of molecular libration. Strain-minimization calculations based on semi-empirical potential functions have been carried out and the molecular parameters so derived are compared with the experimental values.