We have measured the drift mobility of anions in low-density hydrogen cyanide (HCN) gas. They are formed by attachment of photoinjected excess electrons to HCN molecules. Mobility isotherms for the temperatures 294, 333, and 373 K were determined in the HCN number density range 9.36x10(17)less than or equal to n less than or equal to 1.21x10(20) cm(-3). The density-normalized mobility (mu(anion)n)of the anions shows a rather strong density dependence. At least in the limit of very low density the value (mu(anion)n)(0) of some postulated anions can be tested by the statistical adiabatic channel model (SACM) for ion-dipole molecule capture processes published recently by Tree [J. Chem. Phys. 105, 6249 (1996)]. The observed density variation can be explained by cluster anion formation. But additionally one has to consider the influence of HCN dimer formation in the drift gas on the anion mobility. It is further shown that the experimental mobilities recently obtained for cluster ions like NO+(CH3CN)(2) and NO+(CH3CN)(3) in acetonitrile are in total contradiction not only to our experimental results but also to theory.