Sulfur K-edge X-ray absorption spectroscopy (XAS) has been used to investigate the distribution of sulfur types in two whole blood cell samples, in selected subcellular blood fractions, and in cell-free plasma from the tunicate Ascidia ceratodes. Whole blood cells are rich in sulfate, aliphatic sulfonate, and low-valent sulfur. The sulfur K-edge XAS spectrum of washed blood cell membranes revealed traces of sulfate and low-valent sulfur, but no sulfate ester or sulfonate. Sulfonate is thus exclusively cytosolic. Cell-free blood plasma contains primarily sulfate sulfur. Gaussian fitting and sulfate quantitation for two whole blood cell samples, each representing dozens of individuals from separate collections taken per annum, yielded average sulfur type concentrations for the two populations: (first year, second year): [sulfate], 110 mM, 150 mM; [sulfonate], 99 mM, 70 mM, and; [low valent sulfur], 41 mM, 220 mM. On titration of sulfate with acid or V(III) in aqueous solution, the strong 2482.4 eV 1s --> (valence t(2)) sulfur K-edge XAS transition of tetrahedral SO42- splits into 1s --> a(I) and 1s --> e transitions, because HSO4- and VSO4+ are of C-3v symmetry. Gaussian fits and appropriate comparisons allow the following assignments: (compound/complex, 1s --> a(I) (eV), 1s --> e (eV)): myo-inositol hexasulfate, 2480.8, 2482.8; HSO4-, 2481.4, 2482.7; VSO4+, 2481.2, 2482.9. The energy separating the al and e states of complexed sulfate appears to be solvation dependent. From these studies is derived an explicit inorganic spectrochemical model for biological V(III) and sulfate. The average endogenous equilomer concentrations of sulfate complexed with V(III) and/or H+ within the two blood cell samples are calculated from this model. The results provide a natural explanation for the observed biological broadening of A. ceratodes blood cell sulfur K-edge XAS spectra.