Using a single beamz-scan technique with a 30 ps laser pulse at 640 and 1064 nm, the real and imaginary parts of the third-order nonlinear optical susceptibilities of porphyrin oligomers for different numbers of repetition units in solutions were measured. At a probe wavelength 640 nm, the real parts of&khgr;(3) (Re &khgr;(3))of oligomers which consist of porphyrins bridged by 1,4-phenylene moiety were found to be negatively enhanced significantly compared with that of the monomer. WhileRe &khgr;(3)of the oligomers increased with increasing the porphyrin unit, the value per monomer unit remains almost constant; that is,Re &khgr;(3)is proportional to the number of monomer units. The oligomers in which the porphyrin rings are directly attached together at the meso position exhibited enhanced imaginary parts of&khgr;(3)at 640 and 1064 nm. The enhanced real part was also detected at 1064 nm. These unique properties are discussed in terms of the two photon resonance process. ©1997 American Institute of Physics.