Several silylated derivatives of poly(methylphenylphosphazene) [Me(Ph)PN](x)[RMe(2)SiCH(2)-(Ph)PN](y) [R = CH3, 3; (CH2)(2)CH3, 4; (CH2)(2)CF3, 5; and (CH2)(2)(CF2)(8)F, 6] were prepared via deprotonation of the methyl group of [Ph(Me)PN](n) with n-BuLi followed by the treatment with chlorosilanes. Three new terpolymers, [Me(Ph)PN](x)[Me(3)SiCH(2)(Ph)PN](y)[RMe(2)SiCH(2)(Ph)PN](z) [R = (CH2)(2)(CF2)(8)F; x = 0.44, y = 0.14, z = 0.42, 7; x = 0.46, y = 0.27, z = 0.27, 8; x = 0.33, y = 0.54, z = 0.13, 9], were also prepared via cosubstitution using varying ratios of Me(3)SiCl and RMe(2)SiCl. Structural characterization and purity of each of these polymers were established by H-1 and P-31 NMR spectroscopy, elemental analysis, and differential scanning calorimetry. The gas permeabilities of membranes of these polymers were determined at 35 degrees C and compared to that of the parent polymer [Me(Ph)PN](n), 1, for which permeabilities to N-2, O-2, CH4, and CO2 were 0.6 x 10(-10), 1.8 x 10(-10), 0.7 x 10(-10), and 6.5 x 10(-10) cm(3)(STP). cm(-1). cmHg(-1). s(-1), respectively. The incorporation of the silyl groups generally increased permeabilities relative to [Me(Ph)PN](n) without significant changes in selectivity. Polymers from the cosubstitution reactions had as much as a 15-fold increase in gas permeabilities with selectivity ratios (e.g., alpha(CO2/CH4) = 8.9) similar to those of 1. The trimethylsilyl groups also increased the T-g values of the terpolymers 7-9 relative to the simple derivative 6. The gas permeation properties of these polymers are discussed with respect to the lengths and concentration of the side groups.