The penetration rates of water/ethanol mixtures into silica fibrous assemblies were determined from the change in electrical conductivity due to wetting. Silica fibrous assemblies were treated with gamma-aminopropyltriethoxysilane (APTES) and methyltriethoxysilane (MTES). The advancing contact angles of the water/ethanol mixtures on silanized silica, theta(a), were measured by the Wilhelmy method. The electrical conductivity, lambda, began to increase at time t(0) after the wetting liquid-fibrous assembly contact and reached a constant value. By comparing the experimental lambda - t curve with the curve calculated by the Washburn equation, the capillary distribution of the fibrous assembly was determined. The t(0) value as a measure of the penetration rate increased with increasing volume ratio of ethanol in the water/ethanol mixtures for the untreated and APTES-treated silica fibrous assemblies. This indicates that the addition of ethanol to the wetting liquid decreased the penetration rate. In the case of the MTES-treated silica fibrous assembly, liquid penetration was observed above 30% of ethanol and to decreased with increasing volume ratio of ethanol. On the basis of the Washburn equation, the value of 1/t(0) was plotted against gamma(L) cos theta(a)/eta, where gamma(L) is the surface tension of the liquid and eta is the viscosity of the liquid. As expected, the value of 1/t(0) increased linearly with increasing cos theta(a)/eta.