Abstract: A comparison of air-stability of polyacetylenes(PA) prepared by rare-earth catalyst(Ln-PA) and Shirakawa's catalyst (Ti-PA) through examination of ESR, IR, weight uptake and change of electrical conductivity on long time exposure of these PA films to air is presented. It is found that the cis/trans ratio and conductivity decreased with increasing the exposure time for both types of polymers, but the thermal isomerization rate and the decrease of conductivity of Ln-PA film appeared slower than those of Ti-PA film. Similary, a gradual decrease of conductivity was observed for I₂-doped Ln-PA.Spin concentration calculated from ESR intensity decreased with increasing exposure time in the initial stage of oxidation, then increased slightly and decreased. This seems to suggests the occurence of the oxidation reactions (see p.75 eqs. 1, 2).