Abstract: The Coherent Synchrotron radiation obtained at SOLEIL has allowed, for the first time, measurement of broad band absorption, Fourier Transform spectra in the sub-THz range at 0.001 cm -1 resolution, with a signal-to-noise ratio of 100 attained in a few hours. The technique has been applied to the observation of the pure rotational spectrum of propynal (HC≡CCHO) in the 250 -700 GHz range. The new data have allowed the determination of more precise values for rotational constants and for higher order rotational constants. The potential of the technique for rapid recording of rotational spectra in this energy range is outlined.