A differential kinetic fluorimetric method is proposed for the simultaneous determination of thiamine (vitamin B) and its pyrophosphate ester (cocarboxylase). It is based on the oxidation of these analytes by mercury(II) in a basic medium to yield fluorescent thiochrome. The calibration graph of each analyte is linear in the range 4 × 10−8−3 × 10−5M. The logarithmic extrapolation method is used to resolve thiamine/cocarboxylase mixtures in ratios between 4:1 and 1:15. Results of precision and selectivity studies are reported. Thiamine and cocarboxylase are satisfactorily determined in synthetic multivitamin preparations by the proposed method. A method for the individual determination of cocarboxylase is also described and applied to the analysis of this analyte in commercial pharmaceutical preparations.