Poly(phenylacetylene) (PPA) and poly(p-methoxyphenylacetylene) (PMOPPA) were synthesized by catalytic polymerization of monosubstituted alkynes and were fully characterized by conventional techniques. Linear and third-order non-linear optical properties of polymers solutions were investigated at lambda = 780 nm using a Z-scan set-up equipped with a femtosecond laser source at different operating regimes. A high repetition rate (76 MHz) regime was used to measure the linear and non-linear absorption coefficients by means of thermooptical effects. Values of the linear and non-linear absorption coefficients were alpha = 4.9 x 10(-3) cm(-1) and beta = 1.1 x 10(-11) cm W-1 for PPA and alpha = 2 x 10(-2) cm(-1) and beta = 2.1 x 10(-10) cm W-1 for PMOPPA. Very low repetition rate (14 Hz) excitation was used to evidence the purely-optical non-linear refractive indexes, that were gamma = 6 x 10(-18) cm(2) W-1 and gamma = 11 x 0(-18) cm(2) W-1, respectively, for PPA and PMOPPA. (C) 2001 Elsevier Science B.V. All rights reserved.