The fluid catalytic cracking (FCC) unit is one of the most complex interactive processes in the refining industry, and is difficult to operate and control. This work deals with the regulative control law design for stabilization of the reactor and regenerator temperatures, considering that the kinetics terms are poorly known. The proposed control law makes use of on-line estimates of the input/output modeling errors, obtained from a proportional-integral reduced order observer. The structure of the proposed controller is similar to a proportional-double integral compensator (PI2) such that a new parameterization of the controllers gains is given in terms of the closed-loop and estimation time constants. The performance of the control scheme proposed here is analyzed via numerical simulations.