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Specular reflection spectra are reported for a thermochromic polydiacetylene crystal, ETCD [substituent group – (CH₂)₄OCONHC₂H₅] over the temperature range 23–130°C. A reversible thermochromic phase change occurs at ∼120°C and is accompanied by a 2750 cm⁻¹ blue shift in the reflection spectra. Evidence is presented which strongly suggests that the dramatic change in optical properties of ETCD at the phase transition is due to an acetylene to butatriene transformation in the bonding sequence of the polydiacetylene backbone.