Microencapsulated and microsphere products have become increasingly popular in the pharmaceutical and cosmetic industry. They are used as controlled-release drug systems as well as in diagnostic contrast agents. An example on how these kinds of products can be optimised is presented. The selected product is optimised by the use of a model based on an on-line steady state optimisation concept where the objective is based on the final product characteristic, where the product characteristic is measured in-line. The on-line steady-state optimisation algorithm used is a modified simplex where the production constraints are embedded into the optimisation routine.