The effects of the catalyst-to-oil ratio (CTO), temperature, residence time, and feed composition on the product selectivities and product distribution in fluid catalytic cracking (FCC) will be discussed. Under simulated realistic conditions two distinctive time frames can be identified. In the 'initial time-frame', during the first 50 ms of catalyst-oil contact, radical reactions play an important role, leading to the large extent of unselective conversion of the feed to coke, H-2, and C-1-C-4. The presence of catalyst external surface area enhances radical reactions. Coke deposits mainly on the outer surface of the catalyst particle, thereby reducing its accessibility and apparent catalytic cracking activity. After 50 ms, during the steady-state time-frame', radical reactions play a negligible role and no coke, H-2, and C-1-C-2 are formed. In this stage, P-scission is dominant. A lumped kinetic scheme is proposed to describe the observed phenomena and how gasoline yield penalties could be avoided in commercial risers. Calculations on feed reactivities and mass-transfer have been made in order to describe the processes taking place on a molecular scale. (c) 2005 Elsevier B.V. All rights reserved.