Abstract. Following our previous reports on the spectroscopy and photochemistry of methyl and ethyl ethers of simple acyclic and non-strained cyclic z-0x0 oximes, we have now studied 4-(methoxyimino)-2,2,3,3-tetramethylcyclobutanone (l), of which the ring is severely strained, 3,5,5-trimethyl-4(5H)-isoxazolone (S), of which the (Z)-oxime function is constrained in a five-membered ring, and 6-(benzyloxyimino)-2,2,5,5-tetramethylcyclohexanone (lo), which is a benzyl ether. Upon irradiation of (E)-1 with h 254 nm, the main process is photodissociation, leading to the same type of products as obtained with the six-membered ring homologue 3, but in addition to 2,2,3,3-tetramethyl-4-oxobutanenitrile (13) and 2,2,3,3-tetramethylcyclopropanone 0-methyloxime (21). The triplet-sensitized irradiation of (E)-1, in contrast to that of the higher homologues, leads. in addition to E-Z isomerization, to photodissociation, yielding the same photoproducts as obtained after direct irradiation with 254 nm. The 4-isoxazolone 8 is photostable against radiation of 350 and 300nm. Irradiation of 8 with h 254 nm leads to population of the excited singlet m* state and subsequent photodecomposition as a result of inital N -0 homolysis, the observed products being carbon monoxide, acetonitrile, acetone and small amounts of ethanol. The occurrence of the photodecomposition is in agreement with the theoretical prediction"' that the excited singlet m* (Z)-3-0x0 oxime ethers would give N -0 homolysis, provided that the other route of deactivation via Z -E isomerization, which has a much lower energy barrier on the potential energy surface, cannot take place. The photochemistry of the benzyl ether (Z)-10, of which the intercarbonyl-iminyl dihedral angle is approx. 45" and the x systems of the phenyl and the C = N -0 moieties are perpendicular to one another, with h 350, 300 and 254 nm is different from that of the corresponding methyl ether 3" in two ways. Firstly, on using h 350 and 300 nm decomposition was observed which is, however. slow relative to the Z-E isomerization and, secondly, the photodecomposition products are of a different nature, since the assigned primary and secondary product from 10 are benzaldehyde and 6-(benzoylamino)-2,2,5,5-tetramethylcyclohexanone (32), respectively. Mechanisms have been presented to explain the formation of the photoproducts obtained from 1. 8 and 10.