UV spectroscopy is a powerful diagnostic tool in the study of organic composition and chemical processes in planetary atmospheres such as that of Titan. The Spectroscopie UV d'Int¿reˆt Pr¿biologique dans l'Atmosph¿re de Titan (SIPAT) laboratory experiment has been developed in order to systematically determine at high resolution the absorption coefficients of expected molecular absorbers in Titan's atmosphere, with the goal of interpreting mid-UV albedo spectra. We have studied the mid-UV absorption coefficient of propynenitrile (cyanoacetylene), which is a key compound in prebiotic chemistry. In spite of all the precautions taken during the synthesis of this compound, the samples studied contain impurities. Using complementary techniques, we have been able to characterize the impurities (mainly acetone and acetonitrile) in the propynenitrile gas sample. Thus, we present new absorption coefficients of propynenitrile determined at 298 K and at 0.02 nm resolution between 185 and 200 nm (the wavelength range where acetone presents strong absorption features). A precise determination of impurity mixing ratios affecting previously published spectra leads to a reevaluation and the establishment of associated uncertainty values for HC3N absorption coefficients in the 185- to 230-nm range. Finally, these data have been corrected for an erroneous wavelength calibration implying a 3 ¿ shift. ¿ American Geophysical Union 1995