Polarizability of sulfate dianion solvated in water clusters and in an extended water slab
is investigated by a combination of {\it ab initio} calculations
and classical and Car-Parrinello molecular dynamics. 
Quantum chemistry calculations at the MP2/aug-cc-pvdz level 
are performed for structures obtained from molecular
dynamics simulations replacing water oxygens and hydrogens by fractional point
charges. Sulfate polarizability in bulk water amounts to roughly 7 \AA$^3$.
Despite this large value sulfate clearly prefers interior over surface solvation,
in accord with previous experimental evidence for large aqueous clusters.