Vanadium K-edge X-ray absorption spectroscopy (XAS) studies have been carried out at 10 K on packed whole blood cell samples and on Henze solution from the tunicate Ascidia ceratodes (A. ceratodes). High energy-resolution vanadium K-edge spectra exhibit pre-edge transitions at 5464.9 +/- 0.1, 5466.8 +/- 0.1, and 5468.8 +/- 0.1 eV for both whole blood cell samples and Henze solution. The whole blood vanadium K-edge spectrum is very similar to that of vanadium(III) in aqueous sulfuric acid solution. Both spectra exhibit a feature at 5476 eV indicative of an endogenous vanadium(III)-sulfate interaction. This represents the first direct spectroscopic evidence for intracellular [(VSO4)(H2O)(4-5)](+). Absence of the vanadium(III)-sulfate feature in the vanadium X-ray absorption edge spectrum of Henze solution indicates loss of the sulfate ligand on dilution of the intravacuolar contents following whole cell lysis. Vanadium K-edge EXAFS analysis of the whole blood samples revealed about six nearest neighbor oxygen (or nitrogen) atoms at 1.99 +/- 0.02 Angstrom. No evidence either for the more distant carbon shells of an intracellular vanadium chelate or for (VOV)(4+) dimers, was found in the EXAFS spectra of whole blood cells. Since the XAS spectra were taken at 10 K, the likelihood that significant amounts of such species remained undetected is remote. Taken together, the results are consistent with the proposition that blood cell vanadium, at least within the tunicate A. ceratodes, is >90% monomeric, existing as a mixture of the hexaaquovanadium(III) ion and the [(VSO4)(H2O)(4-5)](+) complex ion. The possible presence of up to 10% of, e.g., a tris-chelated tunichrome-vanadium(III) complex is not excluded. Within Henze solution, vanadium K-edge and EXAFS spectral analysis likewise indicated unoxidized, monomeric V(III) with six to seven first shell oxygens at 1.99 +/- 0.02 Angstrom but with no indication of a sulfate interaction.