This paper deals with the preparation of colloidal suspensions of mullite and magnesium-aluminate spinel via a cheaper precursor of alumina sol. Viscosity, pH, solid content, DTA, TGA and XRD studies at different temperatures were performed to characterize those two sols. These mullite and spinel sols were used separately as bonding agents in a high-alumina based ultra low cement castable composition prepared by simple tapping technique and their performances have been compared in terms of bulk density, apparent porosity, cold crushing strength, flexural strength, slag corrosion, thermal shock, XRD and SEM reports. The results confirm that the mullite sol excels while the spinel sol degrades the refractory castable quality. © 2002 Elsevier Science Ltd and Techna S.r.l. All rights reserved.