Technical Abstract: The availability of deuterium-labeled nicotinic acid makes stable isotope dilution mass spectrometry (SIDMS) coupled with liquid chromatography (LC) an attractive option for the determination of the water-soluble B-vitamin, niacin, in food samples. We present a method based on acid digestion, solid phase extraction (SPE) with a strong cation exchange (SCX) column, and reversed phase chromatography with a C18 column. Ionization is by positive ion electrospray. Although analysis can be accomplished in selected ion recording (SIR) mode, it is subject to interference problems similar to those found with other LC determinations of niacin. The additional selectivity of multiple reaction monitoring (MRM) mode, however, largely eliminates interference problems. The method is applied to both fortified samples and samples with only endogenous niacin, including milk samples with sub-ppm niacin levels. The method is capable of determining niacin levels even to the ppb range.