Daily average slag and the corresponding hot metal samples were collected on tapping of BF No.III of the Egyptian Iron and Steel Company (EISCO) and analyzed. The temperature of hot metal was measured during metal tapping. The useful volume of the furnace, which was charged with 100% self- fluxing sinter, is 1033m3.The analyses of slag and hot metal together with the metal temperature were used to estimate activity coefficient and activity of silica in the slag according to the statistical theory of regular ionic solutions. The silicate capacity of the slag and distribution of silicon between slag and hot metal were also assessed. The change of silicon activity in hot metal with the concentration was also treated. The activity coefficient of silicon was found to be 1.324. Calculated silicon distribution ratios satisfactorily correlate with the relevant values obtained from the chemical analysis. The effect of temperature on both silicate capacity and silicon distribution is very large compared with the influence of the slag basicity. The best correlation results were achieved when the basicity definition contains CaO and its equivalents of basic oxides.