Abstract: A monitor determines the yields of constituents of a product provided by a fluid catalytic cracking unit (FCCU) receiving fresh feed and recycle feed. The monitor includes sensors providing signals corresponding to sensed operating parameters of the FCCU. Analyzers analyze the fresh feed and the recycle feed and provide signals corresponding to the API gravities of the fresh and recycle feeds and to the viscosities of the fresh and recycle feeds. A circuit provides signals corresponding to the Watson K factors associated with the fresh and recycle feeds and the catalyst in accordance with the signals from the analyzers and sensors. A network provides signals representative of the yields of the constituents of the product from FCCU. Display apparatus provides a visual display of the yields.