The third‐order nonlinear optical susceptibility &khgr;(3)(−3&ohgr;;&ohgr;,&ohgr;,&ohgr;) of propylmethylpolyazine thin films as measured by the third harmonic generation is reported. The &khgr;(3)(−3&ohgr;;&ohgr;,&ohgr;,&ohgr;) values of propylmethylpolyazine that possess statistically alternating methyl and propyl substituents on the carbon atoms along the conjugated chain were found to be 8.0×10−12esu off resonance at 1.5 &mgr;m fundamental radiation. The interesting third‐order nonlinear properties presumably originate from the unique periodicity of the main &pgr;‐electron‐conjugated polyazine backbone.