The control problem of fluid catalytic cracking (FCC) units is a challenging task due to its model complexity, non-linear dynamics, constrained variables and cross-coupling interaction between inputs and outputs. This paper is concerned with the design of multivariable feedback control configurations for composition control at the riser output for FCC units. A linear cascade (master/slave) control configuration is proposed, which leads to asymptotic regulation of the riser output composition (e.g. gasoline yield) about a feasible set point. Sufficient conditions to achieve regulation in terms of the steady-state gain matrix and response time-constants are provided, allowing to obtain a systematic procedure for analyzing multivariable control configurations of complex and interacting processes. Some tuning guidelines issues are discussed. Numerical simulations on a non-linear dynamical model operating in the partial-combustion mode are used to show the effectiveness of several multivariable control configurations under disturbances and uncertainty parameters. (C) 2003 Elsevier B.V. All rights reserved.