We report the direct observation of electron-electron correlation effects in polyacetylene. Pristine 99% enriched13 cis-rich polyacetylene has been investigated by electron nuclear-nuclear triple (double-ENDOR) resonance spectroscopy. We observe an induced negative spin polarization of -/+-0.33. Negative spin densities arise from spin exchange mechanisms which can only result from Coulomb effects. Theoretical analysis of experimental results within the soliton framework yields 3 eV for the effective on-site Coulomb repulsion. © 1985 The American Physical Society.