Broadband near-infrared pulse generated from NOPA (non-collinear optical parametric amplifier) was used for measurement of the spectrum of susceptibility of the third harmonic generation (THG). Corrections for the effects of absorption and refractive index, contribution of non-degenerate third-order nonlinearity, and THG by air, were properly addressed. This multiplex method was applied to a polydiacetylene, poly[5,7-dodecadiyn-1,12-diol-bis(n-butoxylcarbonyl-methyl-urethane)] (4BCMU) in red phase resulting in a good agreement with theoretical prediction. (c) 2006 Elsevier B.V. All rights reserved.