The influence of various factors on the aging of poly(I-trimethylsilyl-l-propyne) (PTMSP) during long-term storage in air was investigated with Fourier transform infrared spectroscopy. Most attention was paid to the differentiation of oxidation, the reduction of the free volume, and the absorption of low-molecular-weight compounds from the environment. IR spectra of PTMSP samples stored from 1-2 months to 6.5 years revealed C=O, C-C, and C-H bands that had been earlier attributed to polymer oxidation products. It was established, however, that these bands completely disappeared from IR spectra of the aged samples after their soaking in ethanol. Spectra of dried residues of ethanol extracts displayed all these bands and did not show any bands of polymer chain fragments. Gas chromatography/mass spectrometry analysis proved that the residues were composed mainly of various dialkyl phthalates. Special experiments showed that PTMSP films easily absorbed di-n-butyl phthalate vapors. IR spectra of the films stored for a long time showed no decrease in double-bond and methyl group bands. It was concluded that the polymer did not undergo oxidation in air at room temperature for at least 6.5 years. The absorption of plasticizer vapors (and possibly other compounds) from the environment and the reduction of the free volume were the main reasons found for PTMSP aging. (c) 2006 Wiley Periodicals, Inc.