The pesticide Kepone, 1,1a,3,3a,4,5,5a,5b,6-decachlorooctahydro-1,3,4-metheno-2H-cyclobuta(cd)pentalen-2-one, the related compounds mirex, kelevan, a monohydro photoproduct of kelevan, kepone alcohol, kepone hydrate, and the mono- and dihydro photoproducts of Kepone hydrate and the mono- and dihydro photoprodcucts of Kepone hydrate in hydrocarbon solution were examined by 13C and 1H nuclear magnetic resonance (NMR). The Kepone photoproducts were isolated directly from the photolysis products for the first time. Their structures were determined unequivocally to be 1,1a,3,3a,4,5,5a,5b-nona-chlorooctahydro-1,3,4,-metheno-2H-cyclobuta(cd)pentalen-2-one (monohydrokepone) and 1,1a,3,3a,4,5,5a-octa-chlorooctahydro-1,3,4-metheno-2H-cyclobuta(cd)pentalen-2-one(dihydrokepone). The NMR data indicate that the major monohydro photoproduct of kelevan is that with the hydrogen substituent at the 3a or 5b position, anti to the OH substituent. In solution, Kepone can exist as a carbonyl from and as its hydrate, a gem-diol. These do not equilibrate at ambient temperatures on the NMR time scale. Without stringent drying, only the gem-diol forms of Kepone and its mono- and dihydro photoproducts are observed. Variable temperature 1H NMR studies of monohydrokepone gem-diol indicated that it does not form intramolecular hydrogen bands, but forms intermolecular hydrogen bonds to other monohydrokepone molecules and to water. This results in dimer formation, with a rapid monomer-dimer equilibrium and proton exchange between monomers, dimers, and water at ambient temperatures.