Objectives: Former investigations have shown that dentin adhesives can prevent root surface caries. The aim of the present study was to determine the caries-protective effect of a dentin bonding system on sound and irradiated root surfaces in vitro. Methods: The root surfaces of 30 freshly extracted caries-free human molars were thoroughly cleaned, thereby removing the cementum. Fifteen teeth were irradiated. The irradiation dose of 60 Gy was fractionally applied over six weeks (2 Gy/day). The teeth were then coated with acid-resistant nail varnish, exposing two rectangular windows. One window served as an untreated control, while the other window was treated with a dentin adhesive (One-Coat-Bond). The specimens were distributed among the following experimental groups: A: irradiated, treated with dentin adhesive; B: irradiated, not treated; C: non-irradiated, treated; D: non-irradiated, not treated. All specimens were demineralized for 6 days with acidified gel (HEC, pH 4.8, 37 degrees C). From each tooth, two dentinal slabs were cut. The depth of the demineralized areas was determined using a polarized light microscope. Results: The coated, irradiated specimens in group A showed lesions with a mean depth of 63 µm (± 11 µm). In group B lesion depth was increased (91 µm ± 6 µm). The non-irradiated specimens in group C and D showed a mean lesion depth of 58 µm (± 14 µm) and 92 µm (± 7 µm). In group A and C the lesion depth was reduced significantly compared to the untreated groups B and D, the comparison of group A and C and between B and D showed no significant differences (pConclusion: It can be concluded that the demineralization of the root surface can be impeded by application of the dentin adhesive tested. In the present study no differences between irradiated and sound root surfaces could be detected. Keywords: dentin adhesive, root caries, irradiation, demineralization, dentin