Process for the preparation of C3 and C4 olefins and gasoline by: (a) contacting in a fluidised bed reactor a light hydrocarbon feedstock with a first catalyst inventory comprising a medium pore size zeolite catalyst, wherein the first catalyst inventory is a fresh catalyst inventory; (b) combining at least part of the catalyst inventory as used in step (a) with one or more catalyst streams to form a second catalyst inventory comprising a medium pore size zeolite catalyst and a large pore size zeolite catalyst; (c) contacting a hydrocarbon feedstock with the second catalyst inventory in a reactor riser to form cracked products.