The fluorescence of neat liquid p-dioxane and p-dioxane-water mixtures has been studied as a function of wavelength in the range 200–110 nm, and in the system under beta irradiation. It is seen that the quantum yield of fluorescence declines from the absorption threshold to the ionization onset (∼ 160–170 nm), because of the increasing importance of the competing decomposition processes. Above the ionization onset, there is a slight increase in the quantum yield of fluorescence as a result of the occurrence of “recombination fluorescence”. However, it is estimated that in this region, neutralization does not always lead to a vibrationally equilibrated excited state. This explains in part why the G-value of thermally equilibrated S 1 states is considerably lower than G(ion)(∼5), under conditions that fluorescence originates mainly from charge recombination. Auxiliary experiments carried out in the gas phase, in an ion cyclotron resonance spectrometer, elucidated the reaction of p-C 4H 8O 2+ ions with p-dioxane molecules. The results indicate that in the gas phase, proton transfer between these reactants is thermoneutral, and occurs with a lowered efficiency (i.e. does not occur at every collision) in contrast with m-dioxane for which the corresponding proton transfer reaction is highly exothermic and k Rn = k Collision . In the liquid phase, proton transfer is unimportant in the p-dioxane system, but probably is the predominant fate of the m-C 4H 8O 2+ ion in m-dioxane. The well known shift of emission to lower energies in p-dioxane-water mixtures has been examined; the shift depends only on the concentration of water, and not on the type or energy of the radiation. In the presence of added 1-methylnaphthalene, it is seen that there is no change in the quantum yield of emission from methylnaphthalene at energies below the ionization onset, but that there is a decrease in the fluorescence yield with increasing water concentration above the ionization threshold. In p-dioxane: H 2O: toluene mixtures, on the other hand, the toluene fluorescence diminishes with increasing water concentration at all energies. These observations are interpreted in terms of changes in the energy level of the electronically-excited p-dioxane-water aggregate and on proton transfer reactions involving dioxane-water aggregates.