Sulfite at low concentrations (≧ 1 mM caused a cleavage of thiamine in vitro depending upon time, pH value, and temperature. Thiamine monophosphate (TMP) and thiamine pyrophosphate (TPP) were also cleaved, but the reaction rates were slower. For the separation and determination of thiamine, TMP and TPP in yeast high-performance liquid chromatography (HPLC) was used. Thiamine and its phosphate esters were converted into the corresponding fluorescent thiochrome derivatives by alkaline oxidation, adsorbed on an amino-phase HPLC column and eluted with acetonitrile/100 mM-potassium phosphate buffer, pH 7.5 (40+60, v/v) as solvent system. In yeast TPP represented about 73% of the sum of all thiamine compounds, thiamine and TMP 26% and 1%, respectively. During long-term incubations of yeast (up to 24 hours) 2 mM-sulfite decreased the intracellular content of TPP and also of ATP depending upon the pH value of the medium. The decreasing effect of sulfite on the ATP content was faster and stronger than on the TPP content. Two TPP-dependent enzymes, pyruvate decarboxylase [EC.4.1.1.1] and transketolase [EC.2.2.1.1] were inactivated by sulfite within 1 hour to 58% and 13% of the initial values, respectively.