The reduced zero-field mobilities of t-C4H9+ drifting in He, N-2, O-2 and H2O are measured by ion depletion arrival time analysis to be 14.8+/-0.6, 3.7+/-0.8, 3.3+/-0.8 and 0.04+/-0.02 cm(2) V-1 s(-1), respectively. The mobilities are relatively constant with varying drift fields in the three nonpolar gases, but increase significantly with increasing drift field in water vapor, demonstrating the large difference between polar and nonpolar interaction potentials on the mobility of t-C4H9+. The results are compared to locked-dipole calculations. The relevance towards better modeling of hypersonic combustion and ion-injection processes is also discussed. (C) 2003 Elsevier B.V. All rights reserved.