We report cross-linked polycarbosilane (CLPCS) films with superior mechanical properties and insensitivity to moisture. CLPCS are prepared by spin-coating and thermal curing of hexylene-bridged disilacyclobutane (DSCB) rings. The resulting films are siloxane-free and hydrophobic, and present good thermal stability and a low dielectric constant of k = 2.5 without the presence of supermicropores and mesopores. The elastic stiffness and fracture resistance of the films substantially exceed those of traditional porous organosilicate glasses because of their unique molecular structure. Moreover, the films show a remarkable insensitivity to moisture attack, which cannot be achieved by traditional organosilicate glasses containing siloxane bonds. These advantages make the films promising candidates for replacing traditional organosilicate glasses currently used in numerous applications, and for use in emerging nanoscience and energy applications that need protection from moisture and harsh environments. © 2012 American Chemical Society.