The report describes the design, operation and use of the Reactive Pollution Environmental Simulation Model (REM). Its main purpose is to explain to the user how REM functions and from experience gained in operation, how it can best be used. It includes (a) detailed instructions for setting up input decks; (b) how REM may be used in performing control strategies; (c) an understanding of the methodology, techniques and calculational procedures employed in REM; and (d) sufficient information to enable program modifications to be accomplished. The above tasks are facilitated by the incorporation of algorithm descriptions, flow charts, complete data input instructions, sample input decks, a glossary of program terms and program listings in this volume. (Modified author abstract)