The ability of mouthrinses, available in the international market, to form reaction products on demineralized enamel (bioavailability test) was evaluated in vitro. Nine mouthrinses purchased in Chile were evaluated; eight formulated with NaF (one containing 100 µg F/mL and seven containing 226) and one with Na2FPO3 (226 µg F/mL as ion FPO3 ). Demineralized enamel slabs (n=15 per mouthrinse) were sectioned; one half was subjected to the assigned mouthrinse treatment for 10 min and the other half was used to obtain baseline data. Loosely bound and firmly bound fluoride formed on enamel were determined with an ion-specific electrode and the values were expressed in µg F/cm2. The concentration of fluoride and the pH of the mouthrinses were previously determined. Concentrations of loosely bound and firmly bound fluoride formed on enamel were independently analyzed by ANOVA and Tukey's test (α=5%). The loosely bound and firmly bound fluoride concentrations formed ranged from 3.2 to 36.2 and 0.4 to 1.7, respectively. Loosely bound fluoride formed on enamel was significantly more effective in discriminating the effect of different commercial mouthrinses than firmly bound fluoride. Mouthrinses with 226 ppm F as NaF and low pH presented significantly greater bioavailability of fluoride on enamel than those with higher pH or lower NaF concentration. The mouthrinse with Na2FPO3 showed low reactivity. Although further studies are necessary, the findings showed that commercial fluoride-containing mouthrinses have important variations in enamel fluoride bioavailability, which may result in differences on anticaries efficacy.