The chemical origins of the Hofmeister series are investigated using infrared photodissociation (IRPD) spectroscopy. IRPD spectra of SO42-(H2O)(n) with up to 80 water molecules have a band at similar to 3710 cm(-1) for n > 43 but not for n <= 43. This band corresponds to outer-shell water molecules with "free" OH groups similar to those at the surface of bulk water. These results indicate that the sulfate dianion has a long-range effect on water structure, and this effect is likely to be an important factor in its Hofmeister behavior.