Sulfur is an essential biological element, yet its biochemistry is only partially understood because there are so few tools for studying this element in biological systems. X-ray absorption spectroscopy provides a unique approach to determining the chemical speciation of sulfur in intact biological samples. Different biologically relevant sulfur compounds show distinctly different sulfur K-edge X-ray absorption spectra, and we show here, as an example, that this allows the deconvolution of the sulfur species in equine blood.