Elastic moduli (E, G, K, ν) of calcium aluminosilicate glasses with <10 mol% of SiO 2 doped with Er 2 O 3 and Yb 2 O 3 , melted in air and under vacuum conditions, have been measured using the pulse echo ultrasonic technique. There was a decrease of the elastic properties, <5%, as rare earth oxide replaces Al 2 O 3 from 0.2 to 1.5 mol% (about 8 wt%). The Young's modulus did not vary within errors of measurements. The Debye temperature varied between 369+/-9 and 352+/-9 K, which is explained assuming that the rare earth ions acts as network modifiers disrupting the glass structure. The undoped calcium aluminate glasses fracture toughness is 1.4+/-0.3 MPam 1 / 2 and the thermal shock resistance is 339+/-102 W/m.