Cis-cisoidal poly(substituted phenylacetylene)s have poor membrane forming ability. To solve this problem, we synthesized and polymerized three phenylacetylenes having a silicon-containing substituent and two hydroxyl groups. The resulting polymers had cis-cisoidal backbone, a high molecular weight, good solubility, and therefore good membrane forming ability. Their plots in a P-O2/P-N2 vs P-O2 graph were very close to or over Robeson's upper bound line. Especially poly(substituted phenylacetylene)s having oligosiloxanyl groups showed ultrahigh P-O2 (>1,000 barrer).