The photochemistry of a number of arylidenecyclopentanone oxime 0 - acetates has been investigated Irradiation in methanol leads to initial E-Z geometrical isomérisation and ultimately to the formation of nitrogen containing heterocycles, via a 6îr-electron photocyclisation process, followed by an elimination. The scope of the reactions has been extended by investigation of substrates incorporating a range of electron-donating aryl substituents The photocyclisations of 2-(4-dimethylaminobenzyhdene)-, 2-(4-hydroxybenzylidene)-, 2-(4-acetoxybenzylidene) and 2-(2,5-dimethoxybenzylidene) cyclopentanone oxime O-acetates and 2-(4-aminobenzylidene) cyclopentanone oxime were investigated. Substitutents in the 3-position of the aryl system may potentially result in cyclisation at either the 2- or 6-positions of the aryl group. In practice cyclisation has been found to be highly regioselective The range of regioselective reactions has been extended by substitution in the aryl 3- position with t-butyl, hydroxy, acetoxy, dimethylamino and ammo groups. The regioselectivity of the cyclisation reaction has also been investigated with a number of disubstituted aryl groups. An oxime O-acetate denvative of phenothiazine, underwent cyclisation as did 2-(3-phenyl-allylidene)cyclopentanone oxime O-acetate. The leaving group in these cyclisation reactions is oxygen-based. A preliminary investigation into a small number of nitrogen-based leaving groups was carried out. Semi-empirical calculations were earned out on a number of 3- substituted oxime O-acetates to investigate the reasons for the observed regioselectivities during photocyclisation.