Local mobility of three substituted polyacetylenes was investigated by means of a quasielastic neutron scattering technique aiming at elucidating the origin of gas permeability of these polymers. It was found that the local flux, which is defined as the product of the relaxation rate Gamma, and the mobile fraction f(m), increases with the gas permeability coefficient. This suggests that the local flux is an important factor controlling gas permeability.