The structure of water at aqueous interfaces is of the utmost importance in biology, chemistry, and geology. Weuse neutron reflectivity and quartz crystal microbalance to probe an interface between hydrophilic quartz and bulkliquid solutions of H2O/D2O mixtures. We find that near the interface the neutron scattering length density is largerthan in the bulk solution and there is an excess adsorbed mass. We interpret this as showing that there is a regionadjacent to the quartz that is enriched in D2O and extends 5−10 nm into the solution. This suggests caution wheninterpreting results where D2O is substituted for H2O in aqueous interfacial chemistry.