The nonresonant third‐order nonlinear optical properties of thin films of poly(p‐phenylene benzobisthiazole) (PBZT) and PBZT/nylon 66 and PBZT/poly(trimethylhexamethylene terephthalmide) (PTMHT) molecular composites have been investigated using picosecond third‐harmonic generation at 1.9 &mgr;m. The measured &khgr;(3)(−3&ohgr;,&ohgr;,&ohgr;,&ohgr;) for the pure PBZT was (1.37±0.27)×10−11esu. The &khgr;(3)of PBZT/nylon composites showed a linear dependence on composition. In contrast, the &khgr;(3)of PBZT/PTMHT molecular composites significantly deviated from this linear dependence, showing enhanced values. No in‐plane anisotropy in any of the films was detected. The optical damage threshold of PBZT was measured to be ≳50 GW/cm2for 30 ps pulses at 1.9 &mgr;m.