Before and after cis-trans isomerization, the observed C-13-NMR chemical shifts of poly(phenylacetylene) (PPA) in the solid state were investigated on the basis of 13C-NMR chemical shift calculations within AM1 for the cis-transoidal and deflected trans-transoidal forms. Two C-13-resonance peaks in the observed CP/MAS C-13-spectrum were assigned theoretically by the C-13 chemical shifts of the main and side chains. After thermal isomerization, the C-13 peak of the main chain for PPA shifted upfield by 3.5 ppm, in contrast to the downfield shift of the C-13 peak for polyacetylene. This upheld shift of trans-PPA largely was attributed. to the increases of the excitation energy from the ground state to the lowest phi(pi-pi*) state in the paramagnetic terms of C-13 chemical shift on the main chain carbons with the increase in deflected angle tau of 0 to 80 degrees. The +/-80 degrees deflected conformation of the trans-transoidal chain due to the cis-trans isomerization was confirmed.