The aim of the study was to evaluate the relevance of cations in different fluoride compounds for their effectiveness as anti-erosive agents. Human enamel samples underwent a de- and re-mineralisation procedure for 10 days. Erosive demineralisation was performed with 0.05 M citric acid (pH 2.3) 6 x 2 min daily followed by immersion in the test solution 6 x 2 min each. Test solutions were: SnCl2 (815 ppm Sn; pH 2.6), NaF (250 ppm F; pH 3.5), SnF2 (250 ppm F, 809 ppm Sn; pH 3.5), amine fluoride (AmF, 250 ppm F; pH 3.5), AmF/NaF (250 ppm F; pH 4.3), and AmF/SnF2 (250 ppm F, 390 ppm Sn; pH 4.2). In the control group no fluoridation was performed. Mineral content was monitored by longitudinal microradiography. Finally, scanning electron microscopy was performed. The highest erosive mineral loss was found in the control group (48.0 +/- 17.1 microm). Mineral loss was nearly completely inhibited by AmF/SnF2 (5.7 +/- 25.1 microm; p < or = 0.001) and SnF2 (-3.8 +/- 14.4 microm; p < or = 0.001) treatments. Groups treated with SnCl2 (17.6 +/- 19.5 microm; p < or = 0.001) and NaF (13.2 +/- 21.7 microm; p < or = 0.001) showed a decrease in erosive mineral loss, AmF (41.6 +/- 16.0 microm) and AmF/NaF (27.7 +/- 28.4 microm) had no significant effect on erosion progression. The results indicate considerable differences between the fluoride compounds tested. Treatment with solutions containing SnF2 was most effective.