A comparative study of the third-order nonlinear optical properties, via the newly developed heterodyned optical Kerr effect (OHD-OKE) measurements. of silver phenylacetylide and related compounds is reported. [AgC equivalent to CC6H5](n) (1) was found to exhibit efficient third-order nonlinear optical susceptibility chi ((3)) of 2.4 x 10(-14) esu. and second hyperpolarizability, gamma of 9.07 x 10(-32) esu. These results are compared with those of two related silver phenylacetylide compounds. namely. a double salt, (,silver phenylacetylide).(silver tert-butylthiolate) [[AgC equivalent to CC(6)H(5)(.)A-S(t-C4H9)](n) complex (2). and a cluster, triphenylphosphine silver phenylacetylide tetramer, [(C6H5)(3)PAgC equivalent to CC6H5](4) (3). as well as that of the related organic polymer polyphenylacetylene (4), These four compounds represent different type,, of phenylacetylide derivatives: 1 is an organometallic polymer, 2 a polymeric double salt. 3 a discrete metal cluster. and 4 an organic polymer. It was found that the third-order optical nonlinear response was enhanced by the incorporation of silver d elections into the delocalized conjugated organic pi system, and its magnitude is highly dependent upon the extent of the pi delocalization. Specifically, the relative magnitudes of chi ((3)) and gamma follow the order silver phenylacetylide polymer (1) > (silver phenylacetylide)(.)(silver tert-butylthiolate) double salt (2) > polyphenylacetylene polymer (4) > tetrameric (triphenylphosphine silver phenylacetylide)(4) cluster (3). The observed trend may be attributed to the decreasing length of pi conjugation. It is interesting to note that the incorporation of Ag(1) into the polymeric framework of polyphenylacetylene enhances the chi ((3)) by 25-fold for the same de-ree of polymerization (n = 7). The signs of chi ((3)) and gamma, which are related to the response mechanisms. were found to be solvent dependent.