The study attempts to investigate the relative predictability of two major regional scale photochemical models by comparing concentration predictions for various atmospheric chemical species. The Regional Acid Deposition Model (RADM) and the Regional Oxidant Model (ROM) are used to simulate a specific ozone episode over the northeast United States. Trends in spatial and temporal distributions of various photochemical species are compared. Further, the ability of the two models to elucidate relationships between precursor species such as NOx and hydrocarbons with secondary pollutants such as O3, PAN, HNO3, and H202 is examined. Results from the initial phase of the study indicate that although the models show similar spatial patterns of species concentrations, systematic differences in their predictions do occur. The differences result from a combination of effects arising from among others, differences in input emissions and meteorology, numerical representation of the processes and differences inherent to the chemical mechanisms and formulation of the models.