Pentadecanoic acid-d29 is intended for use as an internal standard for the quantification of pentadecanoic acid by GC- or LC-MS. Pentadecanoic acid is a 15:0 saturated fatty acid found in esterified form in the lipids of many bacterial species and in the milk fat of ruminants. It has been used as a biological marker for the intake of dairy fat in the assessment of metabolic risk factors.