In the determination of pyrolusite with the Fresenius-Wild method, as modified by Mohr, attention should be paid to the acidity of solution and the intensity of exposure to daylight, which are two important factors in the digesting procedure.It is shown by our experiments that: 1.When the acidity is below 15 N sulphuric acid and in presence of manganese(Ⅱ), oxalate decomposes as the solution is digested on the water-bath.The higher the acidity, however, the less the decomposition of oxalate will occur.As the acidity reaches 15 N or higher, no decomposition can be detected.