The 410-kilometer seismic discontinuity is generally considered to be caused by a phase transformation of the main constituent of the upper mantle, olivine, alpha-(Mg,Fe)(2)SiO4, to beta-(Mg,Fe)(2)SiO4. Recent data show that H2O dissolves in olivine and other nominally anhydrous mantle minerals and that the partitioning of H2O between olivine and beta-(Mg,Fe)(2)SiO4 is about 1:10. Such behavior strongly affects the region over which the alpha to beta phase transformation occurs and hence the seismic discontinuity that results. The observed width of the discontinuity constrains the maximum H2O content of upper mantle olivine to about 200 parts per million by weight.