The nonlinear absorption in a series of compounds has been investigated using the open aperture Z-scan technique. The compounds investigated were diphenyl acetylene, diphenyl butadiene, diphenyl butadiyne, 1,4-bis (phenylethynyl) benzene, bis (4-biphenyl) acetylene, 4,4'-bis (phenylethynyl) biphenyl. Solutions of the compounds were made in chloroform and showed no linear absorption at the laser wavelength. The Z-scan measurements were carried out using 35 ps laser pulses at 532 nm. Nonlinear absorption was observed for all of these materials, and the nonlinear absorption coefficient (beta) has been determined from this data. The nonlinear absorption process for two of the compounds, bis (4-biphenyl) acetylene, 4,4'-bis (phenylethynyl) biphenyl, is most likely caused by a two-photon absorption to a two-photon allowed state. The nonlinear absorption mechanism for the remaining compounds appears to be two-photon absorption followed by linear absorption from this excited state. The refractive nonlinearity has been more difficult to determine, and in most cases appears to be indistinguishable from the solvent contribution.