The third‐order susceptibilities &khgr;(3)(−3&ohgr;:&ohgr;,&ohgr;,&ohgr;) of Langmuir–Blodgett films of a series of octasubstituted metallophthalocyanines have been measured by third‐harmonic generation experiments at 1.064 and 1.904 &mgr;m. For the latter excitation wavelength a marked enhancement of the nonlinearity for the cobalt‐containing compound has been found, and &khgr;(3)values are one order of magnitude larger than for the other systems. The effect has been associated to the role of two‐photon allowed metal (possiblyd−d) transitions in the cobalt derivative. ©1996 American Institute of Physics.