Polyacetylene prepared by the Durham precursor route can be obtained in a highly oriented form if the transformation reaction is carried out with the polymer in a stress field. We report here a comparison of the stretched and unstretched films. For stretched films the resonant Raman phonon frequencies are lower, the DC conductivity is a factor of 1000 higher and the spins detected in ESR experiments have a room temperature linewidth of 4.25 G: Approximately one half the linewidth measured for unstretched films. We interpret these results as indicating that the chain conjugation length in stretched films is significantly longer than in unstretched films. Disorder and chain entanglement in the precursor polymer, which can be reduced by the stretching process, is therefore the principal factor limiting the conjugation length of polyacetylene prepared by this route. © 1985 The Institute of Physics.