Stacking faults on (001) as well as their influence on the deformation and fracture behavior of MoSi2-WSi2 solid-solutions have been investigated by transmission electron microscopy (TEM), compression tests and fracture toughness measurements. Stacking faults are characterized to be of the Frank-type in which two successive (001) Si layers are removed from the lattice, giving rise to a displacement vector parallel to [001]. The n value of the displacement vector (R = 1/n[001]) is found to be 3.12 ± 10.10 and 3.34 ± 10.10, respectively, for MoSi2 and WSi2, indicating the occurrence of dilatation of the lattice in the direction perpendicular to the fault. The strength of MoSi2 increases with the increase in WSi2 additions, but the deformability declines rather rapidly with the increase in WSi2 additions. The improvement in fracture toughness of MoSi2 is found to be difficult to achieve simply by forming solid-solutions with WSi2 because of the rather rapid decrease in the deformability and the gradual increase in the stacking fault density with the increase in WSi2 additions. © 2001 Elsevier Science B.V. All rights reserved.