Third‐order optical susceptibilities (&khgr;(3)) of poly{(7‐oxo‐7, 10H‐benz[de]imidazo[4’,5’:5,6]benzimidazo [2,1‐a]isoquinoline‐3,4:10,11‐tetrayl)‐carbonyl}, referred to as BBL, and poly(6,9‐dihydro‐6,9‐dioxobisbenzimidazo[2,1b:1’,2’j] benzo[1mn]‐[3,8]phenanthroline‐3,12‐diyl), known as BBB, have been determined using the technique of degenerate four‐wave mixing (DFWM) at 1.064 &mgr;m. BBL has a &khgr;(3)xxxx=(1.5±0.4)×10−11esu, three times as large as that of BBB and 38 times as large as that of CS2. Electrochemical doping led to a small (<30%) increase in the third‐order susceptibility of BBL.