Objectives: The objective of this study was to evaluate the onset of initial demineralization in human dentin pretreated either with two different dentin bonding agents or a desensitizer. Methods: Twenty-eight freshly extracted human molars were included in this study. The root surfaces were thoroughly cleaned, thereby removing the cementum. From each tooth four root dentin specimens were prepared. The specimens were distributed among the following experimental groups: A: Syntac Classic, B: XenoIII, C: Hyposen, D: control group, untreated. One dentin specimen of each group was inserted into both buccal aspects of each fourteen intraoral mandibular appliances. The appliances were worn by fourteen persons for five weeks day and night. One side was brushed daily with a fluoride-containing toothpaste (Aronal). On the other side, plaque was allowed to grow. Individual oral hygiene techniques were performed without any fluorides. During meals, the appliance was stored in 10% sucrose solution. After the in situ period, slabs (150 microns) were ground and studied using a polarized light microscopic. Results: Concerning lesion depth, ANOVA revealed significant differences between brushed and unbrushed specimens. In the brushed groups following lesion depths were evaluated (mean values and standard deviation in microns): Group A: 27.6 (±9.6); group B: 28.7 (±9.6); group C: 34.3 (±20.6); group D: 74.1 (±18.5). The unbrushed specimens showed following lesion depths: Group A: 59.9 (± 13.1); group B: 52.5 (± 12.1); group C: 72.9 (± 19.9); group D: 101.7 (±23.9). Compared to the untreated control, lesion depths in group A, B and C were significantly decreased in the brushed and also in the unbrushed subgroups (pConclusions: Within the limitations of an in situ study it can be concluded that the demineralization of the root surface can be impeded by application of dentin adhesives and desensitizers under different oral hygiene conditions. Schlagwörter: root dentin, sealants, adhesives, demineralization