The U.S. Environmental Protection Agency and the Electric Power Research Institute, the Ontario Ministry of the Environment, and the Atmospheric Environmental Services have funded the development of two complex regional scale Eulerian acid deposition models, RADM and ADOM. There is a consensus among the scientific community that the models should be evaluated as to their skill in predicting output parameters and to establish their credibility. The paper describes the measurements planned in a large field study to provide data for this evaluation. The plan is divided into two parts: operational and diagnostic. The operational evaluation assesses the models' ability to predict air concentrations and deposition patterns. The diagnostic evaluation is an assessment of the behavior of the model and its individual components, transport, dry deposition, gas phase chemistry, emission inventory, etc. Plans for an extensive network of ground sampling stations and intensive aircraft measurements over a two-year period are discussed.