In search of new materials for oxygen-enrichment membranes, high polymers of acetylenes with bulky substituents [2-octyne \(\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{1} \), 2-decyne \(\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{2} \), t-butylacetylene \(\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{3} \), 1-phenyl-1-propyne \(\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{4} \), and 1-chloro-2-phenylacetylene \(\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{5} \)] were synthesized with Mo- or Ta-based catalysts, and their permeability coefficients to oxygen (PO2), nitrogen (PN2), and carbon dioxide were determined at 25°C. Polymer membranes of the aliphatic acetylenes (\(\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{1} \)-\(\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{3} \)) were highly permeable to oxygen. In particular, poly \(\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{3} \) showed a large PO2 value in the order of 10−8 [cm3 (STP) · cm/cm2 · sec · mmHg] together with a fairly high permselectivity for oxygen (PO2/PN2∼3). Polymers of the aromatic acetylenes (\(\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{4} \) and \(\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{5} \)) were less oxygen-permeable, giving PO2 in the order of 10−10, slightly larger than that for polystyrene.