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The complex optical index refraction of an iron (II) spin-crossover coordination polymer is measured in the Terahertz frequency range by Terahertz time-domain spectroscopy (THz-TDS). By scanning the temperature from 288 to 333 K, we have recorded the evolution of the THz spectrum within the low spin - high spin thermal hysteresis loop. We were able to simultaneously infer the refractive index and absorption variations. The low spin-high spin transition has a marked spectral signature in the millimeter wavelength. In the 0.1–0.6 THz frequency range, the variation of real and imaginary part of the index of refraction is 6% and 20%, respectively. A marked absorption is observed above 500 GHz. The THz-TDS provides a clear direct fingerprint of this class of materials, which are interestingly potential candidates for optical data storage and processing devices.