INTRODUCTION In 1977@ the U S Congress declared the conservation and restoration of visual air quality in certain scenic areas to be a national goal The Environmental Protection Agency (EPA) responded with a strategy which begins with the control of visible plumes in the vicinity of point sources As a result of these developments@ the permitting process for a major new plant or operation must now include an evaluation of the visual impact of its plume. Against this background@ several groups have developed computer models designed to predict the quantitative effects of specified emissions on the optical environment To provide a basis for the evaluation of plume visibility models@ the U.S EPA and the Salt River Project (SRP) in 1979 cosponsored the VISTTA program of intensive field measurements at SRP's Navajo Generating Station in northern Arizona. The present report presents results from a cooperative model intercomparison study based on these measurements Four models were evaluated in the Intercomparison Study the ERT Visibility Model@ the Los Alamos Visibility Model@ PHOENIX (Radian Corporation)@ and PLUVUE (Systems Applications Incorporated) All but PHOENIX are fully documented Each model was run by its developers on a common data base furnished by the study coordinator Runs were designed both to test the model as a whole and to isolate the performance of the optics module The four models are similar in concept@ but differ significantly in their emphasis and level of detail. The Los Alamos and PHOENIX models incorporate the most explicit simulations of atmospheric chemistry and optics@ while the more parameterized ERT and PLUVUE models are substantially easier to run The Navajo Generating Station is a modern plant@ its three 750 mW units equipped with efficient electrostatic precipitators and firing low-sulfur coal Navajo was chosen for study as representative of plants already in existence The VISTTA measurement program was specifically designed to provide data for the validation of plume visibility models@ through the use of instrumented aircraft to map the distribution of particles and trace gases along telephotometer sight paths Measurements were earned out under conditions favorable to the formation of a visible plume@ this meant that observations were usually made in the morning (before convective breakup)@ against unclouded sky@ away from the sun Under these conditions@ the plume appeared as a thin brown streak just above the horizon