The crystal structures of silver(I) sulphate, Ag2SO4, have been investigated as a function of temperature. A main feature is the phase transition from the low-temperature ordered phase, F ddd, to the high-temperature disordered phase, P63/mmc. In particular, the high-temperature structure is solved from single crystal synchrotron X-ray measurements. In this phase the title compound undergoes a colossal (anisotropic) thermal expansion of 136(4)x10-6K-1. This is presumably owing to a high anisotropic vibration state of one of the two crystallographically independent Ag-atoms. Simultaneously occurring high ionic conductivity may be associated with silver ions moving along the c-axis using a paddle-wheel assisted percolative mechanism. Onset of metallic silver in the single crystals is documented, seemingly dependent on thermal pre-history, mosaic structure and chemical synthesis. Possible mechanisms explaining this effect, comprising disproportionation or photo-decomposition, are suggested.