The origin of the organic material, and the mechanism of formation of the organo-U complex, in thucholite from the Besner Mine, Parry Sound District, Ontario, have been investigated. Results indicate that thucholite was formed by the action of aqueous solutions containing organic material, possibly as an oil-in-water emulsion. U, Pb, and other relatively soluble oxides were leached from uraninite while Al, Fe, Mg, and silica were deposited in the thucholite. The separation of U from Th and the rare earths is believed to have occurred by oxidation of tetravalent U to the more soluble hexavalent oxidation state. Oxidation of U and the organic fraction of thucholite in an otherwise reducing environment is assumed to be due to the action of radiation decomposition products of water, the hydroxyl radical and/or hydrogen peroxide. Oxidation of the organic material may also be due to subsequent reaction with hexavalent U. The liquid and solid hydrocarbons, by virtue of their optical activity, are shown to be of plant or animal origin, rather than polymerization products of methane and other low molecular weight gases. The hydrocarbons are probably not contemporaneous with the organic fraction of thucholite.