The nitrile compounds HCN and C2HCN (cyanoacetylene) were detected on Titan by Voyager 1. The ethylene-analog nitrile to cyanoacetylene C2H3CN (acrylonitrile) remains undetected. More recently, ground-based observers have detected HCN on Neptune and placed an upper limit on C2HCN. Kinetic aspects of the formation of acrylonitrile have been studied in a discharge-flow mass spectrometric system at T=298 K. The fractional yield for the formation of acrylonitrile from the fast gas phase reaction CN+C2H4→C2H3CN+H has been determined to be (0.2¿0.1). The other possible gas phase reaction leading to the formation of acrylonitrile, HCN+C2H3→C2H3CN+H, has been determined to be slow under planetary conditions at k(T=298 K)=2--7¿10-14 cm3 molecule-1 s-1, but remains an important chemical sink for HCN. The planetary implications of the results are discussed in terms of the ratio of acrylonitrile to cyanoacetylene on Titan and Neptune. ¿ American Geophysical Union 1992