Aluminum doping in tricalcium silicate, Ca-3(SiO4)0, has been studied by high-resolution laboratory X-ray powder difrraction and the Rietveld method. Two nominal series have been designed and studied. Oxygen-fixed Ca3-xAlx/2(Si1-xAlx/2O4)O series has been prepared as single-phase up to x=0.03. However, oxygen-variable Ca3(Si1-xAlxO4)(1-x/2)square(x/2) series has not been stabilized for any composition. The samples show oxide anion conductivity with a small p-type electronic contribution under oxidizing conditions. Typical total conductivities for these solids are 10(-5)-10(-4) S cm(-1) at 1100 K. The oxide ion transference numbers are very high, similar to 0.98, under reducing conditions, i.e. dry 5%H,-N-2,/air gradient. The oxide ion transference numbers are slightly lower, similar to 0.91 under oxidizing conditions, i.e. dry O-2/air gradient. These compounds display a very important proton contribution to the overall conductivities under humidified atmospheres. The proton transference number ranges between 0.72 and 0.55 at 873 and 1023 K, respectively. (c) 2007 Elsevier B.V.. All rights reserved.