It may be advantageous in chemical reactor design to specify hollow porous catalyst pellets instead of solid pellets of the same superficial shape. Design charts and calculation procedures are presented here that enable quick preliminary estimates to be made of whether hollow pellets should be specified in preference to solid pellets. There are two main features to consider. The first is that hollow pellets have a greater area in contact with the process fluid than do solid pellets with the same superficies. This makes the interior catalytic sites of a pellet relatively more accessible when the resistance to diffusion within the pellet is high. When, on the other hand, the resistance to diffusion within the pellet is low, there is no advantage because the interior catalytic sites are then easily accessible but there is less active catalytic material in a given bulk volume. The second feature is that a packed bed of hollow pellets impedes the flow of the process fluid less than would one packed with the corresponding solid pellets. This means that sometimes the effectiveness of the catalyst can be increased and at the same time the reactor pressure drop decreased by choosing hollow catalyst pellets rather than solid pellets. By contrast, when the catalyst effectiveness is increased by choosing smaller pellets the pressure drop is increased.