Changes in static dipole moment and polarizability upon electronic excitation are determined from electroabsorption spectra of unoriented substituted polyacetylenes: poly[(o-isopropylphenyl)acetylene] (PPPA) and poly[[(o-trimethylsilyl)phenyl]acetylene] (PMSPA). The electroabsorption spectrum agrees well with the sum of the first and second derivatives of the absorption spectrum. The dominant contribution of the second derivative is interpreted by the linear Stark effect induced by the permanent dipole moments, which are associated with the charge-transfer transition from the side groups of the main chain. Large static dipole moment differences were obtained to be \Delta mu\ = 74 +/- 10 and 57 +/- 10 D for PPPA and PMSPA, respectively.