Nafion/SiO 2 , Nafion/[ORganically MOdified SILicate (ORMOSIL)] and Nafion/dimethylsiloxane hybrids were created via in situ sol-gel reactions for tetraethoxysilane, diethoxydimethylsilane, and their mixtures. Differential scanning calorimetry studies showed a broad endotherm for unfilled Nafion-H / at T a É 215ЊC that shifts upward for the Q : D Å 1 : 0 (mol : mol) [Q Å Si(O 1/2 ) 4 , D Å (O 1/2 ) 2 Si(CH 3 ) 2 ] hybrid, then shifts downward with decreasing Q : D. This endotherm likely arises from release of H 2 O molecules bound to GSi{OH groups and condensation reactions among silanol groups. The decrease in T a is rationalized in terms of an increasing fraction of flexible D units that disrupt hydrophilic Q structures. T m shifts to lower temperatures with decreasing Q : D, and it is suggested that main chains are restricted by side chains embedded in silicon oxide nanoparticles, but D unit insertion causes side chains to be anchored less strongly. Thermal gravimetric analysis indicates that the first mass loss step for Nafion-H / shifts to higher temperatures as D : Q increases; an increase in D unit fraction inhibits Q unit degradation by evolved HF. A dynamic mechanical transition at T a may arise from side chain motions, and the increase in T a in passing from unfilled Nafion-H / to the 1 : 0 hybrid is due to side chain immobilization by their entrapment in silicon oxide domains. The progression 0 : 1 r 1 : 2 r 1 : 1 r 2 : 1 generates increasing mechanical tensile strength and decreasing ductility; strength enhancement might be due to entanglements between ORMOSIL and pure silicon oxide phases and side chains. Liquid sorption experiments quantified the affinity of these hybrids for solvents of varying polarity. A dielectric relaxation for the 0 : 1 hybrid at about 1.5 kHz might be related to side chain mobility. A weaker relaxation in the range 10 4 -10 5 Hz exists for the 0 : 1 and 1 : 1 hybrids and Nafion-H / , but not for the 1 : 0 hybrid that exhibits the behavior 1Љ Ç f 0n over a broad frequency ( f ) range, suggestive of intercluster proton hopping.