The geometric structures of Cr(NO3)(3) and Zn(NO3)(2) aqueous solutions in a wide range of concentrations 2.7-0.005 m have been determined by means of the extended X-ray absorption fine structure (EXAFS) technique. X-ray absorption spectra at the K-edges of Zn and Cr have been measured in the transmission and fluorescence modes at the Synchrotron Radiation Source (U.K.). The analysis of all the experimental data (13 solutions) is compatible with a unique structural model, which basically agrees with the concentric shell model of Frank and Evans for ionic hydration. Therefore, it is shown that the EXFAS technique allows the determination of a second hydration shell in a wide range of concentrations, from almost saturated to highly dilute solutions. M-O distances (Cr-O-I = 2.00 Angstrom, Zn-O-I = 2.05 Angstrom) and the coordination number (6 for both cations) for the first hydration shell are not affected by concentration. A second hydration shell is detected in both cases, although for chromium solutions, this contribution to the EXAFS spectra is more important than for zinc. The distance is around 4.0 Angstrom, but in Cr3+ solutions a slight increase in the Cr-O-II distance is observed with dilution (4.02 Angstrom for 0.01 m, and 3.95 Angstrom for 2.6 m). The Zn-O-II distance shows no systematic trend, the average distance being 4.1 Angstrom. The coordination number for this shell is 13.3 +/- 1 for Cr3+ solutions and 11.6 +/- 1.6 for Zn2+ solutions. The most concentrated Zn2+ solution (2.7 m) presents a singular behavior, since its coordination number decreases to 6.8 +/- 1.5. The data analysis procedure is thoroughly described, and the possibilities of an alternative hypothesis for the second contribution to the EXAFS spectrum, such as multiple scattering effects, are carefully discussed.