Hyroxyl resonance fluorescence was used to study the reaction of OH with HCN in a discharge-flow system. The effect of side reactions involving OH was assessed by numerical integration of the rate equations for the system. The rate constant for the reaction: OH + HCN → products as measured at pressures above 10 Torr and under conditions where side reactions were believed to be unimportant, is given by the expression: k = 1.60 ° 10-11 T-1exp(-1.86 × 103/T) cm3 molecule -1 s-1 with an estimated error of � 20% over the temperature range from 298 to 563 K.