Background: This in vitro investigation shows how 3.3% H2 O2 , at different pH-values affects the enamel. Material and Methods: A number of fifteen human premolars were used. The enamel of the coronal half in six of the teeth, were exposed by H2 O2 . Nine teeth were prepared to enamel powder. The enamel was exposed to 3.3% H2 O2 , at six different pH-values (pH range 4.5 - 7.0). Analyses of the topography of enamel performed by scanning electron microscope (SEM) and the chemical composition of enamel by X-ray microanalysis (XRMA). X-ray powder diffraction (XRD) analysed the crystallinity in enamel powder . Results: The exposure to H2 O2 at pH<5.5 resulted in a rougher topography of the enamel, according to the SEM studies. The XRMA analysis revealed a increase in the ratio of Ca:C. Exposure to H2 O2 at pH>5.5 resulted in a decrease of O in the exposed enamel, and changes in C:P, Ca:C, Ca:P and Ca:O were observed. The H2 O2 2 did not affect the unit cell parameters, but the signal-to-noise level was increased for slightly acidic or neutral solutions. The unit cell parameters decreased in the acidic solutions. Conclusions: The exposure to H2 O2 at varying pH values affect the enamel with two different mechanisms. One effect is the oxidation of the organic or bioorganic matter in the hydroxyapatite matrix, due to the use of 3.3% H2 O2 . The other effect is due to the current pH of the H2 O2 , since the structure of the hydroxyapatite starts to erode when the pH<5.5.