Objective: To examine the effect of Opalescence (Ultradent, USA) on the demineralisation behaviour and the surface morphology of human enamel. Methods: For the demineralisation study 25 intact enamel surfaces of extracted third molars were cut into two equal halves (test and untreated control half). The enamel of 7 teeth was slightly polished with Soflex disks prior to halving. The test halves were exposed to Opalescence (10% carbamide peroxide) for a total of 96 hours. The bleaching gel was renewed twice daily (8 times in all). Following on the test and control surfaces artificial lesions (0,1M Na-acetate buffer (pH 4,9) in 6% HEC, 7 days) were created in previously prepared windows (1x3 mm). Longitudinal 100µm-sections were produced after demineralisation. The depth of the lesions was estimated under polarized light after imbibition in ethanol. For the morphological study a further 5 surfaces were investigated in a scanning electron microscope (SEM). Epoxy resin replicas of these surfaces were prepared before and after bleaching to enable the investigation of one and the same place. Results: A distinct change in colour of the bleached specimen was evident. In the natural enamel surfaces no differences were found regarding lesion depth of the bleached (56µm) and non-bleached (54µm) specimen. The lesion depth of the polished enamel was significantly greater, again without differences between bleached (76µm) and non-bleached (74µm) specimens. Comparison in SEM showed some alterations at the surfaces after bleaching. A slight local roughening, additional or enhanced micro cracks and a loss of material along natural edges or along traces and defects caused by use were provable at higher magnifications (2000 – 8000x). Conclusion: Although the demineralisation behaviour was not influenced the observed microstructural alterations should be considered seriously in using bleaching agents.