The series of Gd4-xMxAl2O9-x/2 (M = Ca, Sr) with x =0, 0.01, 0.05, 0.10 and 0.25 was prepared by the citrate complexation method. Both Gd4-xCaxAl2O9-x/2 and Gd4-xSrxAl2O9-x/2 show the monoclinic cuspidine structure with space group of P2(1)/c up to 0.05-0.1 and 0.01-0.05 mol for Ca and Sr, respectively. Beyond the substitution limit of Gd4Al2O9, GdAlO3 and SrGd2Al2O7 appear as additional phases. The highest electrical conductivity obtained at 900 degrees C yielded sigma = 1.49 x 10(-4) S/cm for Gd3.95Ca0.05Al2O8.98. In comparison, the conductivity of pure Gd4Al2O9 was sigma = 1.73 x 10(-5) S/cm. The conductivities determined are in a similar range as those of other cuspidine materials investigated previously. The thermal expansion coefficient of Gd4Al2O9 at 1000 degrees C was 7.4 x 10(-6) K-1. The phase transition between 1100 and 1200 degrees C reported earlier changes with increasing substitution of Ca and Sr. (C) 2009 Elsevier B.V. All rights reserved.