Silver photodiffusion is one of the attractive photo-induced changes observed in amorphous chalcogenides. In this research, we focus on amorphous S-rich germanium sulphide and study the kinetics of the silver photodiffusion by neutron reflectivity, as well as optical reflectivity. It was found from the neutron reflectivity profiles with 30 s time resolution that silver dissolved into the germanium sulphide layer, forming a metastable reaction layer between the Ag and the germanium sulphide layers, within 2 min of light exposure. Subsequently, silver slowly diffused from the metastable reaction layer to the germanium sulphide host layer until the Ag concentration in both layers became identical, effectively forming one uniform layer; this took approximately 20 min. Optical reflectivity reveals the electronic band structure of the sample, complementary to neutron reflectivity. It was found from the optical reflectivity measurement that the metastable reaction layer was a metallic product. The product could be Ag8GeS6-like form, which is regarded as the combination of GeS2 and Ag2S, and whose backbone is composed of the GeS4 tetrahedral units and the S atoms. We attribute the first quick diffusion to the capture of Ag ions by the latter S atoms, which is realised by the S–S bond in amorphous S-rich germanium sulphide, while we attribute the second slow diffusion to the formation of the Ag–Ge–S network, in which Ag ions are captured by the former GeS4 tetrahedral units.