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The work described in this thesis concerns the photochemistry of substituted aromatic compounds. In a previous study by the present author of the photonucleophilic substitution of monosubstituted benzenes the scope of the photo-substitution was investigated, but the mechanism was unclear. This work has been continued and the mechanism established. A radical process is excluded, and it is found that the reaction proceeds by attack of a nucleophile on the excited state of the aromatic compound, probably to give a sigma bonded intermediate (i), which decomposes to the substituted product. A series of kinetic experiments established that the nucleophile participates in the rate determining step, but for alkoxide ions the order of the reaction in alkoxide ion is not unity. This is interpreted as being doe to association of the ions to ion pairs and greater aggregates In the non-polar alcoholic solvent. A correlation was noted between the photochemical lability of substituents (where these are leaving groups in the context of nucleophilic substitution) and the development of a partial positive charge on the carbon atom bearing the substituent in the π, π* excited state of the aromatic compound, as calculated using an H. M. 0. approximation. An attempt was made to apply the same considerations to photonucleophilic substitution in the disubstituted benzene series. Although calculations indicated which compounds might be expected to be photochemically reactive, experimented difficulties precluded the formulation of substitution rules. In the monosubstituted naphthalene series the fluoronaphthalenes underwent photonucleophilic substitution readily, but the methoxynaphthalenes gave complex mixtures of unstable substances. In the case of 1-fluoronaphthalene a novel photochemical transformation product of the starting material, 3, 4-benz-2-fluro-tricyclo- (3, 1, O, 02, 6)-hex-3-ene (ii), was isolated. This is the first example of a benz-benzvalene. Irradiation of nitrobenzene with diethylamine was found to give, not the substituted product, N, N-diethylaniline, but reduction products, mainly aniline. A study of the reaction was made, and it was found that photo-reduction of aromatic nitrocompounds is a general phenomenon, in solvents bearing hydrogen atoms capable of abstraction. Interposition of a Pyrex glass filter between the lamp and the photolyte causes coupled species, initially the azoxycompound, to be formed as major products, when a basic solvent is used. However, the azoxycompound is not usually isolated, for it is transformed photochemically both to the o-hydroxyazocompound and, by photosensitization by the starting material, to the azocompound. Mechanistic studies on the photo-reduction of nitrobenzene indicate that reduction occurs by hydrogen abstraction processes, rather than by oxygen atom transfer or electron transfer. Although nitrosobenzene and phenylhydroxylamine have not been isolated from the reaction mixtures, their presence has been inferred, and it is suggested that they are removed from the solution, before they attain isolable concentrations, by reduction by the first formed intermediate C6H5NO2H. This intermediate is seen as arising by hydrogen abstraction from the solvent by excited nitrobenzene in a triplet π, π* state. Although hydrogen abstraction is not a reaction generally associated with π, π* states, H. M. O. calculations on nitrobenzene indicate that in this state an electron-deficiency develops on the oxygen atoms of the nitro group, giving them something of the character of an alkoxy radical. Photochemical studies on azoxycompounds shew that the well- known rearrangement to give the o-hydroxyazocompound (iii - iv) proceeds faster in polar solvents, perhaps suggesting an ionic process rather than a hydrogen atom abstraction/radical recombination. Sensitization encourages photo-reduction to the azocompound; this indicates that rearrangement is probably a reaction of the singlet state, whereas photo-redaction involves the triplet. The situation therefore parallels that found for nitrocompounds, where photo- reduction proceeds via the triplet state and the rearrangement of o-methynitrobenzenes to o-nitrosobenzyl alcohols (v - vi) is a function of the singlet state. Furthermore, just as the normal photo-reaction of o-nitrobenzyl compounds (reduction) is faster than the oxygen transfer rearrangement, so in the azoxycompounds, oxygen transfer to a phenyl ring is a process much preferred to oxygen atom transfer to an ortho methyl group (vii - viii).