This paper presents the synthesis of copolymers of vinyl p-t-butylbenzoate (PTBBA) with 3-methacryloxypropyltris(trimethylsiloxy)silane (Si-1) and their permselective properties for oxygen and nitrogen. Analysis of the glass transition temperature profile by the Kwei equation implied the existence of some interaction between PTBBA and Si-1 moieties. The oxygen permeability of 1.6 x 10(-8) cm3(STP)cm cm-2 s-1 cmHg-1 for the Si-1 homopolymer decreased with decreasing Si-1 content to arrive at 9.1 x 10(-10) cm3(STP)cm cm-2 s-1 cmHg-1 for the PTBBA homopolymer. Oxygen and nitrogen permeabilities were investigated in connection with Si-1 content and the glass transition temperature in the copolymer. Furthermore, the effect of the condition of preparation on gas permeability was studied; that is, we prepared the copolymers of PTBBA and Si-1 with various polymerization times, and investigated their permselective properties for oxygen and nitrogen.