Maintenance policy optimization is concerned with determining when and to what extent major plant equipment items are to be taken off production for preventive maintenance. This paper considers the problem of determining optimal preventive maintenance policy parameters (e.g. effectiveness, frequency) for different items of equipment in multipurpose plants. Since preventive maintenance activities tend to be infrequent, a long-term view of the operation of the process must be taken. The procedure followed involves the formulation of a long-term combined production/maintenance aggregate planning problem, where the effect of failures on plant profitability are taken into account using the measure of expected resource availability as developed for continuous plants and modifying its use for applicability to the multipurpose mode of production. A variety of ways of formulating the problem as mixed integer non-linear or mixed integer linear programs are explored, and two examples are used to illustrate the ideas developed.