Third-order harmonic generation (THG) and electroabsorption (EA) in substituted polyphenylacetylenes (PPAs) which are examples of one-dimensional pi-conjugated polymeric systems have been studied. The values of chi((3))(-3 omega;omega,omega,omega) obtained from THG measurement for substituted PPAs increase with increasing lambda max of absorption. The dispersion of chi((3)) (-omega;omega,0,0) for PPA film derived from Kramers-Kronig transformation of optical absorption change under the electric field (EA spectra) has been observed, and it shows the similar shape of the second derivative of the optical absorption spectra. The typical value of chi((3))(omega;omega,0,0) for o-substituted PPA was determined to be ca. 10(-11) esu.