Linear alpha,omega -vinyl-terminated poly(diethylsiloxane) (PDES) chains were synthesized and fractionated to obtain melts of low polydispersity. The fractionated samples were characterized using gel permeation chromatography and dilute solution viscometry. PDES elastomers were also prepared from the melts using hydrosilylation end-linking. The effects of chain stiffness on solution viscometric properties and on network swelling in toluene were compared to previously obtained results with poly(dimethylsiloxane) (PDMS). The mechanical behavior of the PDES elastomers in uniaxial extension was studied with a Perkin-Elmer dynamic mechanical analyzer. The dry netmorks exhibit a stress-induced amorphous-to-mesophase transition, which results in a discontinuous stress-strain relationship. The effects of molecular-level structure on network mechanical properties were deduced by studying PDES elastomers with well-defined molecular weight between chemical cross-links.