In this paper, the activity coefficient of aluminum in liquid iron with reference to pure liquid aluminum as the standard state is calculated from existing thermal and distribution data with certain assumptions. First, the silver-lead equilibrium diagram is used to obtain the activity coefficient of silver in liquid lead. Next, the data of Wright and of Lorentz and Erbe on the distribution of silver between liquid lead and aluminum are combined with the foregoing to obtain the activity coefficient of silver in liquid aluminum and therefore aluminum in liquid silver. Chipman's data on the distribution of aluminum between liquid iron and silver are then utilized to obtain the activity coefficient of aluminum in liquid iron. Finally, the aluminum deoxidation constant is calculated and compared with the experimental value obtained by Gokcen and Chipman.