The Regional Oxidant Model (ROM) determines hourly concentrations and fates of ozone and 34 other chemical species over a scale of 1000 km x 1000 km for ozone 'episodes' of up to one month's duration. The model structure, based on phenomenological concepts, consists of 3 1/2 layers. The surfaces separating the layers respond to variations in space and time in the meteorological phenomena simulated in each layer. The model simulates many physical and chemical processes that affect the motion and distribution of chemical concentrations; among these are: horizontal transport, photochemistry, nighttime wind shear and nocturnal jet; cumulus cloud effects and mesoscale vertical motion; terrain and mesoscale eddy effects; subgrid scale chemistry processes, natural sources of hydrocarbons, NOx and stratospheric ozone; and dry deposition. The ROM is a complex model that requires users to have expertise in photochemical grid modeling. Meteorologists, engineers, and computer scientists familiar with this type of modeling will find the User's Guide relevant and helpful for running the ROM.