This paper presents the results of a detailed experimental study of the diffusion of propene in large crystals of DDR zeolite (the pure silica analog of ZSM-58) carried out by interference microscopy (IFM). Diffusion is very slow as the propene molecules are only just small enough to pass through the distorted 8-ring windows of the DDR structure. This makes it possible to measure many concentration profiles during the available time and thus to study the diffusional behavior at a level of detail that would be impossible in faster systems. In an ideal DDR crystal diffusion should occur only in the radial direction. In the actual system radial diffusion was dominant but there is also a small but significant axial component which complicates the interpretation of the experimental data. The diffusivity increases strongly with loading but the apparent magnitude of this effect may be increased by the small contribution from axial diffusion. The activation energy for radial diffusion is about 34 kJ/mole. (C) 2015 Elsevier Ltd. All rights reserved.