When the acid mine drainage water, formed as the result of massive pyrite oxidation in the flooded Smolnik mine, mixes with surface water of Smolnik Stream, characteristic brown-ochre precipitates are formed. These ochres continue to form in the strong acidic aqueous environment: stream water pH values are in range 3–5, and ochre deposit pore water has a pH< 3. The ochre samples have been studied by X-ray diffraction and electron microscopy methods (TEM, SEM) with elemental microanalysis. Large amounts of the fresh precipitates are X-ray amorphous. The SEM and TEM investigations have shown that there are a few mineral crystalline phases in ochre hardpan samples consisting of crystals some tens to several hundred nm large in size. The Fe-sulphates, jarosite (KFe₃(OH)₆(SO₄)₂) and probably schwertmannite (Fe₈O₈(OH)₆S0₄) are found in the precipitates and have formed from high SO₄^2- and acid solutions. Goethite (alpha-FeOOH) has also been identified in the ochre deposits, and is the dominate mineral when the pH value increases and the S0₄^2- concentration decreases in the water-ochre deposit system. In acid conditions aluminosilicate minerals have been dissolved or degraded and amorphous Si-Al phase and possibly neoformed halloysite have formed as a result.