Photoreactions in the gas-phase complexes Mg center dot+(1,4-dioxane) (1) and Mg center dot+(1,3-dioxane) (1M) have been examined in the wavelength region of 230-440 nm. Photoproduct assignments are facilitated with the help of deuterium substitution experiments. The main energy relaxation channel for both photoexcited complexes is the evaporation of Mg center dot+. Also observed from I are rich photoproducts with m/z 28, 41, 54-58, 67, 69, and 88; the most abundant one at m/z 54 is designated to Mg center dot+(O=CH2). In marked contrast, the photolysis of 1M yields only Mg center dot+(O=CH2) other than Mg center dot+. Density functional calculations are performed to obtain optimized geometries and potential energy surfaces of 1 and 1M. Although Mg center dot+(chair-1,4-C4H8O2) (1a) and Mg center dot+(boat-l,4-C4H8O2) (1b) are comparable in energy, the much better agreement of the experimental action spectrum of Mg center dot+(1,4-C4H8O2) with the calculated absorption spectrum of la than with that of 1b indicates the predominance of la in the source due to the stability of the chair-1,4-dioxane. For photoreactions, the C-O bond is found to be much more prone to rupture than the C-C bond due to the coordination of O to Mg+ in the parent complexes. Photoreaction mechanisms are discussed in terms of two key insertion complexes, which rationalize all of the observed photoproducts.