Poly(ethylene glycol) (PEG)/polydimethylsiloxane (PDMS) hydrogels were synthesized by cross-linking norbornene end-functionalized polymers with a tetrafunctional thiol using thiol-norbornene chemistry. The swelling capacity and mechanical properties, including the Young's modulus (E) and fracture toughness (G(c)), of the hydrogels were characterized and quantified as a function of the volume fractions of PEG and PDMS. E and G(c) increased simultaneously with the volume fraction of PDMS. The moduli of the hydrogels were quantitatively described and predicted as a function of the volume fraction ratio of PEG to PDMS using the Voigt and Reuss models. The fracture toughness was well described by the Lake-Thomas theory at low volume fractions of PDMS. As the volume fraction of PDMS increased, PDMS the hydrogels but also contributed to hydrogel toughness. not only controlled the swelling capacity of the hydrogels but also contributed to hydrogel toughness.