Concentrated (0.1-1.0 kmol m-3) aqueous solutions of YCl3 and ErCl3 have been studied by the anomalous X-ray scattering (AXS) technique at the Y K-absorption and Er LIII-absorption edges, respectively. The Y3+ ions are found to have 8.1+or-0.3 and 8.2+or-0.5 water molecules with the average Y3+-H2O distances being 0.246+or-0.002 and 0.251+or-0.002 nm for 0.5 and 1.0 kmol m-3 YCl3. Similarly, the Er3+ ions are surrounded by 8.2+or-0.6 and 8.3+or-0.8 water molecules with the average distances being 0.240+or-0.002 and 0.238+or-0.002 nm for 0.5 and 1.0 kmol m-3 ErCl3. These results show that the hydration structures around Y3+ or Er3+ ions in YCl3 or ErCl3 solutions resemble each other. In the present study, the limit of the concentration of cations applicable to the AXS measurements has also been verified.