Owing to the fact that Mn availability in soils varying in pH and Eh,the values of available Mn determined through soil tests tend to be an irreliable indicator for the actual Mn availability to crops in the field In literature,however,there is some incorrect interpretation and confusion in discussion of the relationships between soil pH and Mn availability This study examined the solubility of manganese dioxide,in the form of birnessite(MnO₂),in three acids(HCl,HNO₃ and H₂SO₄) varying in concentration and its mechanism,and the reduction powers of three halides(KCl,KBr and KI) under two different pH conditions The effect of pH on the solubility of MnO₂ was elucidated in theory and experiment Results showed that MnO₂ could be dissolved instantly by HCl high in concentration In concentrated HNO₃ and H₂SO₄,however,this was not observed A process of reduction by water took place slowly in the two acids and the reaction was still not completed in a two-year standing When the three acids were low in concentration,none noticeably dissolved the MnO₂ with the H⁺ ion strength <0.5 mol L^-1 HCl significantly dissolved much more MnO₂ than HNO₃ or H₂SO₄ when the H⁺ ion strength reached>1.0 mol L^-1 The reduction powers of the three halides were observed as KI>KBr>KCl and further enhanced by a decrease in pH and an increase in concentration KCl had a very weak reducing power at pH>3,while KI showed a very strong reducing power even at the very low concentration(0.001 mol L^-1) regardless of pHs under the test The above results further indicate that H⁺ perse,regardless of its concentration,cannot reduce MnO₂ in the absence of reductant or e^--donor On the contrary,MnO₂ can be reduced in large quantity in the presence of strong reductant even at a higher pH Low Eh and pH are most favorable to reduction of MnO₂ to Mn²⁺.