Differential scanning calorimetry (d,s.c.) confirms the greater stability of deuterated polystyrene compared to normal polystyrene with respect to oxidation in air. However, the same technique shows that the stability of deuterated cis-polyacetylene with respect to oxidation in air is no better than that of normal cis-polyacetylene. Our d.s.c. results also suggest that there may be a weak isotope effect for the hydrogen migration reaction under nitrogen but not for the isomerization of deuterated cis-polyacetylene to the trans isomer.