Sulfur has a particularly rich biochemistry and fills a number of important roles in biology. In situ information on sulfur biochemistry is generally difficult to obtain because of a lack of biophysical techniques that have sufficient sensitivity to molecular form. We have recently reported that sulfur K-edge X-ray absorption spectroscopy can be used as a direct probe of the sulfur biochemistry of living mammalian cells [Gnida, M., et al. (2007) Biochemistry 46, 14735-14741]. Here we report an extension of this work and develop sulfur K-edge X-ray fluorescence spectroscopic imaging as an in vivo probe of sulfur metabolism in living cells. For this work, we have chosen onion (Allium cepa) as a tractable model system with well-developed sulfur biochemistry and present evidence of the localization of a number of different chemical forms. X-ray absorption spectroscopy of onion sections showed increased levels of lachrymatory factor (LF) and thiosulfinate and decreased levels of sulfoxide (LF precursor) following cell breakage. In intact cells, X-ray fluorescence spectroscopic imaging showed elevated levels of sulfoxides in the cytosol and elevated levels of reduced sulfur in the central transport vessels and bundle sheath cells. ; Ingrid J. Pickering, Eileen Yu Sneeden, Roger C. Prince, Eric Block, Hugh H. Harris, Gregory Hirsch and Graham N. George ; Copyright © 2009 American Chemical Society