The carbon number of the main product and the total yield of products increased with an increase in the amount of triethylamine (TEA). Furthermore, the decrease of DL-2-C-hydroxymethyl-3-pentulose (2-H-3-P) was speeded up by increasing the TEA concentration and 2,4-bis(hydroxy-methyl)-3-pentulose (2,4-BH-3-P) increased smoothly along with the progress of the reaction. In the low formaldehyde (HCHO) concentration range (ca. 0.5 M), dihydroxyacetone (DHA) and DL-glycero-tetrulose were main products. 2-H-3-P and 2,4-BH-3-P increased with an increase in the formaldehyde concentration. Dihydroxyacetone, DL-glycero-tetrulose, 2-H-3-P and 2,4-BH-3-P were favorably obtained from a formose reaction by choosing a suitable [thiamine. HCl]/[HCHO] ratio. Under the reaction conditions reported in this paper, thiamine decomposed rapidly and lost its catalytic ability.