The rebuilding of the reactor for a cat cracker with incorporation of a set of new process components and equipment subassemblies, which are directed toward a reduction in the portion of nonselective secondary reactions taking place in the separation and stripping sections of the reactor, is described; this has made it possible to improve the productivity and effectiveness of the cat cracker.A combined G-43-107M/1 cat cracker has been in service at the Ufa Oil Refinery since 1955. During its construction, the reactor of the cat cracker (CC) was rebuilt on the basis of technology provided by one of the foreign companies [1].From 1997 through 2004, studies were conducted to improve the process and equipment in the reactor unit of the CC, which had included the installation of highly effective feedstock and slime injectors in the straight-run reactor, modernization of the stripping section of the reactor with the assembly of cascading trays and refined vapor-distributing devices, and the installation of tubular air distributors and highly effective regenerator cyclones [2, 3].Since 2004, the cat cracker has been operating on four-year inter-overhaul cycles, the first of which was successfully completed in 2008. Equipment inspection of the reactor unit of the CC after such an extended run indicated that it is in good condition, and does not require any significant repair or restorative work.The vacuum gasoil of a mixture of Western Siberian and Bashkir crudes (the content of the latter attains 45 wt. %) with an end boiling point of 560-570°C is refined in the combined cat cracker. Up to 15 wt. % of heavy viscosity-breaking gasoil is drawn into the CC. After hydrofinishing under comparatively mild conditions, the density of the feedstock ranges from 908 to 912 kg/m 3 , and the carbon residue (Conradson carbon) from 0.3 to 0.5 wt. %.The characterization factor of the hydrofinished feedstock does not exceed 11.7; this permits it to be classified as highly aromatized, and difficult-to-crack. In refining such a feedstock prior to rebuilding of the reactor, the regeneration temperature was close to the limiting allowable value of 730°C.