The role of water, initially mixed with air, upon the properties of shock compressed air has been investigated using a thermodynamic equilibrium computer program. The principal effect of the inclusion of water in the computation of the shock parameters of air is to reduce the temperature of the products. This reduction in temperature results in a reduction in the equilibrium electron concentration by reducing the ionization of NO. The maximum effect occurs at shock velocities of 4.0 to 4.3 km/sec. A procedure is presented for estimating the effect of the addition of water.