The oxidation of poly(1-trimethylsilyl-1-propyne) (PMSP) proceeded normally in air at room temperature and under mild thermal conditions. Two different bands assigned to the carbonyl groups on a side chain and on a decomposed chain end were found in the IR spectra of the oxidized PMSP membrane. However, no band assigned to a hydroxyl group was observed. An allyl-type methyl group of the PMSP was mainly autooxidized without a decrease in molecular weight at room temperature. Only the former band assigned to the carbonyl groups appeared and it had a shoulder peak. In the case of mild thermal oxidation, the decomposition started in the backbone chain and both bands appeared. When the thermal oxidation proceeded further, all bonds were broken and various carbonyl and siloxane bonds were then randomly produced.