The dissolution of sugar-coated tablets of thiamine disulfide largely depended on the lag time, which was greatly affected by the pH of the medium, and was accelerated by mechanical destructive force. The disintegration time-pH profiles were very similar to the dissolution rate-pH profiles determined by using a disintegration test device. Markedly slow dissolution and disintegration of one of the tablets at pH 7.2 was attributed to the dissolving characteristics of polyvinylacetal-diethylaminoacetate (AEA^[○!R]) applied to the tablet as a coating film, and the slow dissolution of another tablet at pH 3-5 was attributed to the use of 2-methyl-5-vinylpyridine-methylacrylate-methacrylic acid copolymer (MPM^[○!R]). Six products were tested for bioavailiability. Statistically significant differences were found in bioavailability among the products. Human gastric acidity greatly affected the bioavailability of the tablet that disintegrated poorly at pH 7.2, and the bioavailiability was significantly lower in subjects with low gastric acidity than in those with high gastric acidity. The in vivo parameters of high and low acidity subjects correlated well with the in vitro parameters determined at pH 1.2-5 and pH 5-7.2, respectively.