In general, the chemistry of seawater experimentally reacted with basalt is in accord with the observed chemistry of the 350°C vent waters from 21°N on the East Pacific Rise. Experiments at 350°C, 500 bar, and a water/rock ratio of ≤10 reproduce most of the major components in the vent waters, in particular the low Mg and SO4 and high Si02, Ca, and K. In comparison with the vent waters however the experimental fluids are deficient in H+ and heavy metals. Experiments with natural and evolved seawater (Mg and SO4 depleted) at ≥400 C and 500 bars or 375°C and ~375 bars under rock-dominated conditions more closely reproduce the vent-water chemistry.