Stochastic simulation can be used to account for uncertainty in the design and operation of chemical processing systems. By using random numbers from probability distribution functions as uncertain input parameters, the system response is studied. This paper is concerned with the application of gPROMS, a new combined discrete/continuous modeling system, to stochastic simulation. The application of the principles of stochastic simulation to chemical engineering are shown and possible simulation modes are identified. The new random number generation module providing random functions for gPROMS is presented. It is based on random streams, allowing the use of more than one random sequence within the same simulation. Three examples of steady state and dynamic stochastic simulations are presented.