The research work described in this thesis has been separated into two parts and an appendix. These contain reports on the crystal and molecular structures of dimethyl ketene dimer and acedianthrone, determined by X-ray diffraction methods. The appendix describes a further analysis of the results of the three dimensional investigation of naphthalene, undertaken to obtain more information on the positions of the hydrogen atoms and on the effect of the pi electrons on bond length. The unit cell dimensions and space group of fluoranthene have also been determined.