Natural abundance 13C NMR experiments using cross polarization and magic angle spinning are reported for polyacetylene as a function of AsF5 doping level. In contrast with previous authors we find that the doping of cis-polyacetylene leads, even at low doping levels, to a new signal characteristic of the doped regions of the polymer. Similar measurements on trans-polyacetylene indicate that the doping of the trans isomer proceeds considerably more uniformly than that of the cis isomer. Comparisons with appropriate model compounds suggest that the observed shift of the 13C NMR signal on doping is primarily a chemical shift rather than a Knight shift. © 1982.