Owing to their combined open-framework structures and semiconducting properties, two-dimensional thiostannates show great potential for catalytic and sensing applications. One such class of crystalline materials consists of porous polymeric [Sn₃S₇ ²⁻] _n sheets with molecular cations embedded in-between. The compounds are denoted R-SnS-1, where R is the cation. Dependent on the cation, some R-SnS-1 thiostannates transition into amorphous phases upon dispersion in water. Knowledge about the fundamental chemical properties of the thiostannates, including their water stability and the nature of the amorphous products, has not yet been established. This paper presents a time-resolved study of the transition from the crystalline to the amorphous phase of two violet-light absorbing thiostannates, i.e. AEPz-SnS-1 [AEPz = 1-(2-aminoethyl)piperazine] and trenH-SnS-1 [tren = tris(2-aminoethyl)amine]. X-ray total scattering data and pair distribution function analysis reveal no change in the local intralayer coordination during the amorphization. However, a rapid decrease in the crystalline domain sizes upon suspension in water is demonstrated. Although scanning electron microscopy shows no significant decrease of the micrometre-sized particles, transmission electron microscopy reveals the formation of small particles (∼200–400 nm) in addition to the larger particles. The amorphization is associated with disorder of the thiostannate nanosheet stacking. For example, an average decrease in the interlayer distance (from 19.0 to 15.6 Å) is connected to the substantial loss of the organic components as shown by elemental analysis and X-ray photoelectron spectroscopy. Despite the structural changes, the light absorption properties of the amorphisized R-SnS-1 compounds remain intact, which is encouraging for future water-based applications of such materials.