Acid mine drainage (AMD) is a particular focus of environmental research, involving the examination of many pollutants and their reactivity. Special attention must be paid for an accurate characterization of AMD samples in the field and in the laboratory. AMD samples are typically unstable due to the presence of colloidal material such as iron oxyhydroxides and bacteria. They induce physical and chemical transformations that may affect the analytical results. Variations in pH are recorded within days following sample collection, indicating changes in their chemistry. The present work demonstrates evidence of the instability of AMD samples, through the results of monitoring experiments performed under different laboratory conditions. Expeditious indicators, such as pH and electrical conductivity (combined with the analysis of crystallinity, composition and morphology of the resulting precipitates), allow monitoring of the evolution of the samples. The results also provide information about the transformation of the typical ochre products that precipitate from AMD, suggesting the formation of schwertmannite and its evolution to a more crystalline mineral.