The ratio, D(T)/K, of the transverse diffusion coefficient to mobility for Na+ ions drifting in Xe gas has been experimentally measured. Inevitable longitudinal end effects present in the drift tube were duly corrected for. Results at approximately 303 K are reported at electric field to neutral gas density ratios, E/N, ranging from 5 to 440 Td. Furthermore, elaborate calculations of the reduced mobility, K0, the ratio, D(L)/K, of the longitudinal diffusion coefficient to mobility, and D(T)/K were calculated with a Monte Carlo simulation (MCS) method which used the interaction potential of Koutselos, Mason, and Viehland V. Chem. Phys. 93, 7125 (1990)] as input. The accuracy of this potential has been evaluated by comparing the results with the experimental data and those calculated using the generalized Einstein relations technique. Agreement within 3% between the experimental values of D(T)/K and those from MCS calculations for the whole range of E/N suggests that the interaction potential represents fairly well the true potential. The error of the experimental measurements of D(T)/K is estimated to be better than 3.5% and that of the MCS calculations is better than 1% for K0 and 2.5% for D(T,L)/K.