A K-edge X-ray absorption spectrum (XAS) fitting approach has been developed to speciate elements of interest in complex materials and used hen to model the storage of biological vanadium within whole blood cells from the tunicate Ascidia ceratodes. The response of the K-edge XAS of solution-phase V(III) to increasing c(sulfate) at constant pH 1.8 produced specific and systematic effects in the preedge transition at 5468.8 eV (preedge transitions: 1s --> (4)A(2) at 5464.9 +/-0.1 eV, 1s --> T-4(2) at 5466.9 +/- 0.1 eV, and 1s --> T-4(1) at 5468.8 +/- 0.1 eV for 11 different V(III)/sulfate solutions). In contrast, variations in acidity (as pH) at constant c(sulfate) systematically modified the V(III) preedge XAS at 5466.9 eV. The energy position of the K-edge absorption maximum also serially shifted -0.32 eV/pH unit, from 5483.7 eV (pH 3.0) to 5484.7 eV (pH 0.3). Fits to the V-IC XAS of two samples of A. ceratodes whale blood cells representing dozens of animals implied storage of V(III) ions in four predominant solution regimes: similar to 10% high sulfate/pH 0 acid; similar to 40% high sulfate/pH 1.8 acid; similar to 40% moderate sulfate/pH 1.8 acid; similar to 10% moderate sulfate/pH 3 acid. For lysed blood cells, the best fit represented 63% of the V(III) in a pH 1.6 sulfate-free environment and a further 16% in acidic sulfate solution. Nearly 18% of lysed cells vanadium(III) appeared in a tris(catecholate)-like environment. A detailed speciation of biological vanadium complex ions was calculated from these fits by application of the known equilibrium constants governing V(III) and sulfate in acidic aqueous solution. The utility of blood cell V(TLI) to ascidians is discussed. Fits to K-edge XAS spectra using the XAS spectra of appropriate models are suggested to be generally applicable to elucidating the state of metal ions in a wide variety of complex environments.