It has been known that caffeic acid and other o-diphenol compounds accelerated the decomposition of thiamin at pH 7.0〜7.5 in vitro. The composition of various thiamin analogues with caffeic acid (A), and of decomposition products of thiamin or thiamin analogues with caffeic acid or catechol (B) were investigated to clarify the mechanism of these reactions. Reaction conditions were as follows: (A): Reaction mixtures contained 3 ml of 1/15M phosphate buffer (pH 7.4), 1 ml of 30 μM thiamin analogues and 1 ml of 30 μM caffeic acid. These mixtures were incubated at 48℃ for 2 hours. (B): Reaction mixtures contained 1.0g thiamin and 0.2 g catechol in a total volume of 10ml. These mixtures were incubated at pH 7.0〜7.4 48℃ for 10 days. Results were as follows: 1. With caffeic acid or catechol: Thiamin, hydroxymethylthiamin, hydroxyethylthiamin, and oxythiamin were decomposed, but the sodium salt of thiol type thiamin and thiamin disulfide (TDS) were not decomposed. 2. These decompositions were inhibited by cysteine or ascorbic acid ratio of more than 10 molar ratio to thiamin. 3. Products of these decompositions were TDS, 2-methyl-4-amino-5-hydroxymethylpyrimidine, thiazol, several unknown products (1, 2, 3, 4 and 5) and thiazolone, thiothiazolone and diazepine (less than 3% of total decomposition products). 4. The unknown products of 2 and 3 were diminished gradually. Then water and methanol insoluble material increased gradually and reached almost 90% of the thiamin after the reaction mixture had stood for 10 days at 48℃.