Scanning electron microscopy and an electron-microprobe analysis of a sample of blast furnace (1317) coke have revealed alkalization (5.64 wt % Na2O + K2O) and Al saturation (17.28 wt % Al2O3) Of SiO2 by BF gases. The K/Na-at value of 1.15 in the new phase (alteration zone) reflects close atomic proportions of the elements and suggests that the abilities to incorporate K and Na during the process are almost equal. This Al saturation and alkalization Of SiO2 indicates an active role for Al along with alkali metals in BF gases. The average width of the altered area in the SiO2 grain is about 10 mu m, which suggests that SiO2 particles of that size can be transformed fully to the new phase, provided that at least one of their faces is open to an external pore (surface of the coke) or internal pore with circulating BF gases. The grains that exceed 10 mu m can only be partly altered, which means that smaller SiO2 grains can incorporate more alkali metals and Al (during their transformation to the A] and alkali-bearing phase) than a similar volume Of SiO2 concentrated in larger grains. Thermodynamic calculations for 100 g(solid)/100 g(gas) and temperatures 800-1800 degrees C have shown that the BF gases have very little or no effect on the alkalization Of SiO2. If the alteration process described in this paper proves to be a generalized phenomenon in blast furnace cokes, then the addition of fine-grained quartz to the surface of the coke before charging a BF can be useful for removing of some of the Al and alkali from the BF gases and reduce coke degradation by alkalis, or at least improve its properties until the temperature reaches approximately 2000 degrees'C.