Development of commercially viable chemical transformations of biomass components into platform chemicals is hampered in many cases by poisoning of the metal catalysts assisting these conversions. Sulfur contained in the small amount of protein in biomass is one type of catalyst poison. An energy dispersive X-ray fluorescence (ED-XRF) spectroscopy method was developed to analyze low ppm level sulfur (S) in biomass feedstocks and in subsequent residues from pretreatment reactions. Calibrations utilized disks made with portions of S-containing standards milled with microcrystalline cellulose. These mixtures represented a method of "standard additions" that gave excellent linear plots from similar to 2 to 2250 ppm and simultaneously determined that Avicel contained 23 (+/- 5) ppm S. Accuracy of this method was verified by two external laboratories each using a different method of analysis, UVF and ICP-AES. A minimum detection limit of 2 ppm was determined for this method with an 86 ppm standard. Representative biomass feedstocks and pretreatment residues were analyzed for S.