Fixed-bed reactors configured to create annular flow contacting within beds of catalyst particles offer several advantages over the conventional plug flow arrangement. In particular, pressure drop is reduced whilst maintaining good reactor productivity. However, whilst there have been myriad studies of the axial plug flow through catalyst beds, much less is known about annular flow contacting so it is difficult to quantify and/or predict behaviour with reaction(s). Electrical tomography has already been used to show that it is straightforward to image tracer mixing patterns in 3-D space and time (Bolton et al, 2004). In this paper it is shown how flexible software can be based on the networks-of-zones concept which is simpler and easier than conventional CFD and moreover is fundamentally suitable for interpreting 3-D concentration fields accompanying tracer mixing tests. This software has in-built flexibility allowing the “deconstruction” of the spatial pattern of radial and axial flow whilst simultaneously predicting the exit RTD. The predictions can be presented in equivalent 3-D tomographic formats using see-through solid-body graphics. Predictions are compared with some experimental images.