The present study evaluated the effect of chitosans with different viscosities, dissolved in an AmF/SnCl2 solution, against erosion or erosion/abrasion. A total of 192 specimens were assigned to 2 × 6 groups (n = 16 specimens each): negative control, 4 chitosan solutions (groups Ch50, Ch500, Ch1000, and Ch2000, with viscosity of 50, 500, 1,000, or 2,000 mPas, respectively, 0.5% chitosan, 500 ppm F-, 800 ppm Sn2+, pH 4.4), and positive control (500 ppm F-, 800 ppm Sn2+, pH 4.3). One half of the groups was demineralized (experiment 1, E1; 10 days, 6 × 2 min/day, 0.5% citric acid, pH 2.8) and exposed to solutions (2 × 2 min/day); the other half was additionally brushed (15 s, 200 g) with non-fluoridated toothpaste before solution immersion (experiment 2, E2). Treatment effects were investigated by profilometry, energy-dispersive X-ray spectroscopy and scanning electron microscopy (SEM). In E1, all the chitosan-containing solutions reduced enamel loss by 77–80%, to the same extent as the positive control, except for Ch2000 (p ≤ 0.05), which completely inhibited tissue loss by the formation of precipitates. In E2, Ch50 and Ch500 showed best performance, with approximately 60% reduction of tissue loss compared to the negative control group (p ≤ 0.05 compared to other groups). SEM analysis showed differences between negative control and the other groups but only minor differences amongst the groups treated with active agents. In both E1 and E2, treatment with active agents resulted in surface enrichment of carbon and tin compared to negative control (p ≤ 0.001); brushing removed parts of carbon and tin (p ≤ 0.001). Chitosan shows different properties under erosive and erosive/abrasive conditions. Under erosive conditions high viscosity might be helpful, whereas lower viscosity seems to be more effective in cases of chemo-mechanical challenges.