A support structure for a structured catalytic packing is disclosed. The support structure is in a fixed position relative to the reactor tube containing it. It supports catalyzed casings that are free to move relative to the support structure. The support structure and casings are inserted in the reactor tube such that the support structure is located proximate the longitudinal axis of the tube and the casings are located between the support structure and the reactor tube wall. The support structure comprises a central support tube or rod proximate to, and impervious or perforated discs perpendicular to, the longitudinal axis of the reactor tube, and may comprise spacers separating the discs.