Thiamin and its respective vitamers, thiamin mono- and di-phosphate, can be easily analyzed using high-performance liquid chromatography-fluorescence methods. By utilizing a 1% w/w solution of potassium ferricyanide, thiamin vitamers can be easily converted to their respective thiochrome esters and analyzed to very low levels in samples. The method described in this chapter can be easily utilized as a base for a variety of samples with high reproducibility and repeatability.