A Monte Carlo study is made of the properties of the defect created by the substitution of porphyrin molecules in the Shpolskii matrix n-octane. The atom-atom potential method is used to model the intermolecular interactions. The theoretical orientational and several other properties of the two energetically most favorable defect sites agree quite well with experimental information. In addition, a possible explanation of the Shpolskii effect in this system is found: The octane lattice is packed in a very interlocking structure and seems to be capable of supporting two or three molecule vacancy cages which do not collapse. These are large enough to hold a porphyrin molecule in such a way that its π-orbital system will be less perturbed by the surroundings than is possible in typical solid environments. © 1980 American Institute of Physics.