Series of high-cis and cis/trans poly[(fluorophenyl)acetylene]s (PFPhA) have been prepared by polymerization of (2-fluorophenyl)acetylene, (3-fluorophenyl)acetylene, and (4-fluorophenyl)acetylene with catalysts: [Rh(1,5-cyclooctadiene) OCH3](2) (high-cis PFPhAs) and tungsten(VI) oxychloride/tetraphenyltin (cis/trans PFPhAs). The molecular weight and configurational stability under various conditions at room temperature were studied for both PFPhAs series by means of size exclusion chromatography, H-1-NMR, and UV-vis techniques. All samples exhibited slow degradation when exposed to the atmosphere in the solid state; the rate of degradation was independent on the F-position on the Ph ring. The rate of degradation increased up to three orders of magnitude in the tetrahydrofuran solution where it was higher for high-cis polymers compared with their cis/trans counterparts. The degradation of high-cis PFPhAs was accompanied by significant cis-to-trans isomerization in aerated tetrahydrofuran solution. Rate of degradation and isomerization exhibited the same dependence on the F-position on the Ph ring. The hypothesis was postulated that the degradation of high-cis PFPhAs in solution was accelerated by cis-to-trans isomerization due to which the content of unpaired electrons on the main chains is enhanced. In both high-cis and cis/trans series of polymers the ortho-substituted isomers exhibited an enhanced stability compared with meta-and para-substituted isomers. (c) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4296-4309, 2010