With synchrotron radiation, we have studied the photoionization and dissociation of 1,4-dioxane/ammonia clusters in a supersonic expansion. The observed major product ions are the 1,4-dioxane cation M+ and protonated cluster ions M(NH3)(n)H+ (where M = 1,4-dioxane), and the intensities of the unprotonated cluster ions M (NH3)(n)(+) are much lower. Fully optimized geometries and energies of the neutral cluster M(NH3)2 and related cluster ions have been obtained using the ab initio molecular orbital method and density functional theory. The potential energy surface of the excited state of M(NH3)(2)(+) was also calculated. With these results, the mechanisms of different photoionization-dissociation channels have been suggested. The most probable channel is electron ejection from the highest occupied molecular orbital, followed by the dissociation into M+ and (NH3)(2). For another main channel, after removing an electron from the second highest occupied molecular orbital, the intracluster proton transfer process takes place to form the stable unprotonated cluster ion M(NH3)H+-NH2, which usually leads to the dissociated protonated cluster ion M(NH3)H+ and a radical NH2. (C) 2004 American Institute of Physics.