The selenogallates CsGaSe3 and Cs2Ga2Se5 release gaseous selenium upon heating. An in situ high-temperature X-ray powder diffraction analysis revealed a two-step degradation process from CsGaSe3 to Cs2Ga2Se5 and finally to CsGaSe2. During each step, one Se-2(2-) unit of the anionic chains in Cs-2[Ga-2(Se-2)(2-x)Se2+x] (x = 0, 1, 2) decomposes, and one equivalent of selenium is released. This thermal decomposition can be reverted by simple addition of elemental selenium and subsequent annealing of the samples below the decomposition temperature. The influence of the diselenide units in the anionic selenogallate chains on the optical properties and electronic structures was further studied by UV/vis diffuse reflectance spectroscopy and relativistic density functional theory calculations, revealing increasing optical band gaps with decreasing Se22content.