Novel tricontinuous membranes consisting of well-defined hydrophilic poly(ethylene glycol) (PEG) and lipophilic polyisobutylene (PIB) segments crosslinked by oxyphilic poly(pentamethylcyclopentasiloxane) (PD5) domains have been synthesized and characterized. Tricontinuity arises because the three membrane constituents-PEG, PIB, and PD5-are mutually incompatible and give rise to three independent cocontinuous phases (channels). The continuous PEG segments impart swelling in water (hydrogel character), the rubbery PIB moieties provide strength, and the PD5 domains provide crosslinking and enhanced O-2 permeability. The synthesis involves the random cohydrosilation of various lengths (number-average molecular weights) of alpha,omega-diallyl-PEG and alpha,omega-diallyl-PIB segments by pentamethylcyclopentasiloxane (D5H) followed by water-mediated oxidation of the SiH groups of the D5H to SiOH groups, which immediately polycondense to PD5 domains. Membranes containing about equal amounts of PEG, PIB, and PD5 give rise to tricontinuous morphologies that allow the simultaneous permeation of water, heptane, and oxygen via three cocontinuous channels. The number-average molecular weight of the PEG segment, that is, the number-average molecular weight of the hydrophilic segment between two PD5 crosslink sites, determines the dimensions (pore sizes) of the channels through which water can permeate. A method has been developed for studying the oxygen permeability of membranes. The microarchitecture of the membranes has been investigated with selective swelling experiments and Fourier transform infrared spectroscopy, their mechanical properties have been examined in the water-swollen state with Instron measurements, and their bulk morphologies and thermal degradation have been determined with differential scanning calorimetry and thermogravimetric analysis, respectively. The findings have been interpreted in terms of phase-separated PEG, PIB, and PD5 microdomains.