Experiments were made on the stabilities of thiamine (B_1) and six B_1-derivatives in ascorbic acid (AsA) solution (50 mg/ml) at pH 4.0,45 and 100℃ in comparison with those in acetic acid solution of the equimolar concentration to the AsA solution at the same pH. Stabilities of B_1 and O-ethoxycarbonylthiamine (OCET) in the AsA solution were similar to those in the acetate buffer. S-Ethoxycarbonylthiamine (CET) was converted into OCET and B_1 both in the acetate buffer and in the AsA solution, and the formation of B_1 was accelerated by AsA. Main product from O, S-bis (ethoxycarbonyl) thiamine (DCET) in the AsA solution was OCET. These findings suggest that CET and DCET are decomposed by AsA mainly through the cleavage of S-ethoxycarbonyl ester linkage. This was supported by the easy formation of O-butoxycarbonylthiamine from S-ethoxycarbonyl-O-butoxycarbonyl-thiamine in the presence of AsA. Thiamine disulfide (TDS) and thiamine propyldisulfide (TPD) were converted to B_1 by AsA, where TDS was less stable than TPD.