Dihydroxyacetone, DL-glycero-tetrulose, and 2,4-bis(hydroxymethyl)-3-pentulose (2,4-BH-3-P) were favorably formed in a formosereaction by choosing a suitable ratio of water to N,N-dimethylformamide used as solvent or a suitable [thiamine·HCl]/[HCHO] ratio. The formosereaction was strongly affected by the concentration of formaldehyde in the water layer. Under the reaction conditions studied, thiamine