The synthesis and photochemistry of a number of arylidenecycloalkanone oxime ethers has been investigated . Irradiation of E,E- 2-benzylidenecyclopentanone oxime O-allyl ether results in rapid E-Z isomérisation around the carbon-nitrogen double bond accompanied by slower E-Z isomérisation around the carbon-carbon doub le bond, yielding four geometrical isomers which have been isolated and characterised . This indicates that isomérisation around the carbon-nitrogen double bond is the more facile process. Prolonged irradiation in methanol leads to the formation of 2,3-dihydro-1H-cyclopenta[b]quinoline, involving a 671-electron photocyclisation process via an unisolated dihydropyridine intermediate followed by elimination of allyl alcohol. The scope of the cyclisation has been investig a ted with a series of arylidenecycloalkanone oxime O-methyl ethers. Substitution at the aromatic ring with a methyl or methoxy group a lso y ields the expectedphotocyc lised product on irradiation, whilst substitution with chloro or nitro groups does not. Irradiation of furylidene and 1-naphthylidene d erivatives yields new heterocyclic compounds and irradiation of the cyclohexanone derivative yields 1 ,2,3,4-tetrahydroacridine.