Monte Carlo computer modeling has been used to study the hydration of sodium montmorillonite clays. In this letter we present our calculated values for the equilibrium clay layer spacing, as the water content is increased systematically from 0 mg/g of clay to 300 mg/g of clay. We then compare our results with experimental clay swelling curves. First, we confirm that our new atomistic model is able to reproduce the equilibrium swelling behavior of real clays. Second, we show that when real clay swelling results in nonequilibrium states, our calculated data are enclosed by the experimental adsorption/desorption hysteresis loops. We conclude that our simulations establish, for the first time, the true equilibrium clay layer spacings of the system.