The drift tube is the most crucial part of an ion mobility spectrometer (IMS), as it is here that the desired separation process of different ionic species takes place. A homogeneous electric field is assumed inside the drift tube, enabling an exact calculation of the mobilities of ionic species formed. For verification of this assumption, the electric field inside a drift tube was examined by means of a field simulation program, using an IMS developed at the Institute of Spectrochemistry and Applied Spectroscopy as an example. A refined design of the drift tube was developed, yielding a dramatic improvement in the resolution achieved. The influences of geometric variations in the drift tube, as well as metallic encapsulation neighbouring the drift tube, on the field homogeneity are estimated.