The physical properties, chemical composition and structure of a carbonaceous deposit formed on a radiantly heated HK40 steel tube at 920 to 930° C after ~ 50 days cracking operations in naphtha-steam have been examined. The deposit consisted of two layers. The innermost (i.e. adjacent to the tube) varied in thickness from 20μm to 1.5 mm, and was formed by heterogeneous reactions catalysed by iron and nickel. Enhanced deposition was characterized by a higher concentration of entrapped chromium rich oxide particles, a principal source of catalytic species. The outer deposit layer was of uniform thickness, had a columnar radial and an axial layered structure, which together with the absence of any significant inorganic constituents, suggested an autocatalytic growth mechanism. The corrosion of the inner (pyrolysis) and the outer (hot box) surfaces of the underlying HK40 steel tube had similar features. The chromium rich outer scales were of variable thickness, due to the attack of certain grains, underlying which were zones denuded in carbon precipitates and intergranular silica formation. The major difference was carburization from beneath the decarburized zone at the inner surface. The processes of decarburization, carburization and of intergranular corrosion were interconnected. Carbon pick-up probably resulted from a crevice corrosion mechanism. A general reduction in the nitrogen level through the cracker tube, with enhanced denitriding at both surfaces, originated from a similar mechanism to that responsible for decarburization. The more severe denitriding at the inner surface was derived from chromium nitride conversion to the more stable chromium carbide.