Preservation of thioarsenates is challenging, especially in the presence of iron. Flash-freezing and cryo-storage, which was considered to be the best available technique, led to arsenic co-precipitation with iron upon thawing and transformation of thioarsenates to arsenite with further oxidation to arsenate. Even in the absence of iron, flash-freezing was found to be problematic when water samples were in disequilibrium. In arsenite solutions with excess sulfide increasing formation of thioarsenates was observed as an artifact upon storage. High ionic strengths were found to have a similar effect of pushing thioarsenate distribution towards equilibrium which renders it difficult to transfer a preservation method developed with synthetic thioarsenate solutions in the laboratory to any type of natural water chemistry. Addition of EDTA to complex excess iron also led to arsenite oxidation and artificial thioarsenate formation. An effective method that required minimal logistic effort was to use anoxic septum vials and store the samples at 4 °C in 1% ethanol to reduce any remaining oxygen. This method preserved (thio)arsenic speciation in both synthetic and natural waters in the absence and presence of iron or excess sulfide up to one week. With the new method we were able to show that iron-rich mineral waters in Czech Republic had a much higher share of thioarsenates (up to 50%) than previously reported and that up to 26% trithioarsenate existed – a species which has not been described from iron-rich waters before.