Alumina and silica sols prepared from cheap precursors have been characterized with respect to viscosity, zeta potential and differential thermal analysis. It has been observed that silica sol possesses higher viscosity and zeta potential values than those of alumina sol. The effect of these sols on high alumina based ULC castables has been investigated by comparing their bulk density, apparent porosity, cold crushing strength, flexural strength, slag corrosion and thermal shock resistance properties. XRD and SEM studies have been carried out on fired samples to explain the effect of sols on the fired strength and homogeneity in structure. © 2003 Taylor and Francis Group, LLC.