The molecular structures of 1,8-diaminonaphthalene (1) and 1-amino-8-triphenylphosphoranylideneaminonaphthalene·0.5 C6H6(2) have been solved by X-ray crystallography. The benzene guest molecule is discussed in relation to the host matrix hole in which it is included. The molecular structure of compound 1 has been compared with the results of a theoretical analysis at the AM1 level. The complete potential-energy surface, as a function of the conformation of both NH2 groups (1, 2), has also been calculated. The absolute minimum (1=2= 47.0°) is very close to the experimental geometries of the two independent molecules of compound 1.