Concentrated tin- and fluoride-containing mouthrinses are effective erosion inhibitors in enamel and dentin. To test whether this is also true for solutions with lower concentrations, we conducted a randomized double-blind three-cell crossover in situ study with extra-orally performed erosive impacts (citric acid, 6 x 5 min/day) and an intra-oral rinsing protocol (1 x 30 sec/day) in 24 volunteers. The mouthrinses were a placebo, a NaF (500 ppm F⁻), and an amine fluoride (AmF)/NaF/SnCl₂ mouthrinse (500 ppm F⁻, 800 ppm Sn(2+)). Compared with the placebo, the NaF mouthrinse reduced substance loss by 19% in enamel and 23% in dentin (p ≤ 0.01 each); the AmF/NaF/SnCl₂ mouthrinse reduced this parameter by 67% in enamel and 47% in dentin (p ≤ 0.001 each). AmF/NaF/SnCl₂ was significantly more effective than NaF in both tissues (p ≤ 0.01). The mouthrinse containing Sn and F exhibited good efficacy, even under severe erosive conditions.