The mobilities of NO+(CH3CN)(n) cluster ions (n = 0-3) drifting in helium and in mixtures of helium and acetonitrile (CH3CN) are measured in a how-drift tube. The mobilities in helium decrease with cluster size [the mobility at zero held, K-0((0)), is 22.4 +/- 0.5 cm(2) V-1 s(-1) for NO+, 12.3 +/- 0.3 cm(2) V-1 s(-1) for NO+(CH3CN), 8.2 +/- 0.2 cm(2) V-1 s(-1) for NO+(CH3CN)2 and 7.5 +/- 0.5 cm(2) V-1 s(-1) for NO+(CH3CN)(3)] and depend only weakly on the characteristic parameter E/N (electric held strength divided by the number density of the buffer gas). The size dependence is explained in terms of the geometric cross sections of the different cluster ions. The rate constants for the various cluster formation and dissociation reactions have also been determined in order to rule out the possibility that reactions occurring in the drift region influence the measurements in the mixtures. Since high pressures of acetonitrile are required to form NO+(CH3CN)(2) and NO+(CH3CN)(3), the mobilities of these ions are found to be dependent on the acetonitrile concentration, as a result of anomalously small mobilities of these ions in acetonitrile [K-0((0)) = 0.041 +/- 0.004 cm(2) V-1 s(-1) for NO+(CH3CN)(2) and 0.044 +/- 0.004 cm(2) V-1 s(-1) for NO+(CH3CN)(3)]. These values are at least an order of magnitude smaller than any previously reported ion mobility, which can be partly explained by the large ion-permanent dipole interaction between the cluster ions and acetonitrile. The remaining discrepancies may be the result of momentum transfer outside the capture cross section, dipole-dipole interactions, ligand exchange, the formation of long-lived collision complexes or the transfer of kinetic energy into internal energy of the cluster ion and the acetonitrile molecule.