An ion soft-landing approach was applied to study the motion of hydronium (D(3)O(+)) and cesium (Cs(+)) ions from 84 to 104 K in glassy 3-methyl-pentane (3MP) films vapor deposited on Pt(111). Both ions were found to have very similar mobilities in 3MP. The span of ion mobilities probed is from approximately 10(-18) to approximately 10(-13) m(2) V(-1) s(-1). Ion transport in these films was studied as a function of film thickness and electric field strength. The drift velocity was found to be linear with applied field below about 2 x 10(8) Vm and deviated from linearity above this. To a large extent, D(3)O(+) and Cs(+) motion in 3MP was well predicted by a simple continuum-based ion mobility model in films from 25 to 20,000 ML thick (including pronounced perturbations 7 ML from both the vacuum and Pt interfaces). The mobility varied with temperature more slowly than predicted by Stokes' law, which may be due to extended inhomogeneous structures in the 3MP near its glass transition at 77 K.