Short-chain fatty acids (SCFA) are produced in the gut by microbial fermentation and are influenced by the microbiota present in the host organism. The Mass Spectrometry Core Facility offers SCFA analysis of acetic acid, propionic acid and butyric acid by GC/MS. The service includes sample extraction with ether, derivatization, and analysis. The method takes advantage of spiked heavy-isotope internal standards for each analyte to correct for potential variability in derivatization. The service yields an absolute concentration for the analytes, expressed in mM. In addition to acetic, propionic, and butyric acids, we have also included lactic acid, succinic acid, valeric acid, isovaleric acid, and isobutyric acids in past analyses.