This paper introduces the Integrated process simulation environment (IPSEpro (TM)) software, and its application to reverse osmosis desalination. The paper introduces the reader to the basic modeling concepts. and how individual process units can be simply simulated with a graphical, flowsheet style interface. The paper then goes on to describe an example simulation for off design of an integrated pump, membrane rack and isobaric energy recovery device, investigating the effects of changing water quality, membrane condition and also, the influence of isobaric energy recovery device overflush. (C) 2009 Elsevier B.V. All rights reserved.