Composite membranes based on a hydrophobic glassy poly[3-(trimethylsilyl)tricyclononene-7] (PTCNSi-1) confined in the channels of anodic alumina with different pore diameters are discussed. Formation of continuous polymer film with partial penetration of polymer into the rigid pores of anodic alumina was achieved by spin-coating technique under vacuum suction. Mass-transport characteristics of composite membranes reveal a slight reduction of composite membrane permeability for condensable gases, and many-fold permeability drop for permanent gases as compared to the bulk film. This results in an ideal selectivity rise over 35 for C 4 H 10 /CH 4 pair compared to 12.6 for bulk PTCNSi-1. The effect is attributed to a solubility-controlled mobility of polymer segments confined in the AAO channels and formation of rigid shallow polymer layer at AAO/polymer interface, which suppress transport of gases. The correlation between intrinsic properties of the polymer (hydrophobicity, Kuhn segment) and its transport characteristics in the confined state is discussed. The evolution of the permeance and pure-gas selectivity of the composite membranes during ageing is also reported.