ALTHOUGH THE VOYAGER GALILEO AND CASSINI MISSIONS HAVE PROVIDED US WITH UNPRECEDENTED INSIGHT INTO THE THREE-DIMENSIONAL STRUCTURE AND COMPOSITION OF THE ATMOSPHERES OF JUPITER AND SATURN WE STILL DO NOT DEFINITIVELY KNOW THE SOURCE OF THE UPPERTROPOSPHERIC HAZES ON THESE PLANETS OR THE PHYSICAL AND CHEMICAL DRIVERS RESPONSIBLE FOR THE OBSERVED VARIATIONS IN ATMOSPHERIC COMPOSITION AS A FUNCTION OF ALTITUDE AND LATITUDE. OUR THEORETICAL UNDERSTANDING OF TROPOSPHERIC CHEMISTRY ON THE GAS GIANTS IS PARTICULARLY POOR. WE PROPOSE TO ADDRESS SOME OF THESE DEFICIENCIES BY DEVELOPING THEORETICAL MODELS FOR JUPITER AND SATURN THAT WILL ILLUMINATE THE UNDERLYING PROCESSES CONTROLLING THE TROPOSPHERIC GAS-PHASE COMPOSITION AEROSOL STRUCTURE VERTICAL AND MERIDIONAL DISTRIBUTION OF CONSTITUENTS AND SEASONAL VARIATIONS IN THE ABOVE PROPERTIES. WE WILL USE PHOTOCHEMISTRY/TRANSPORT MODELS THAT EXTEND THROUGH THE UPPER TROPOSPHERE AND STRATOSPHERE TO INVESTIGATE THE PHOTOCHEMICAL COUPLING OF AMMONIA PHOSPHINE HYDROCARBON AND OXYGEN SPECIES ON BOTH JUPITER AND SATURN. THE PHOTOCHEMICAL MODEL RESULTS CONCERNING THE PRODUCTION RATES OF CONDENSABLE PHOTOPRODUCTS WILL BE COUPLED WITH AN AEROSOL-MICROPHYSICS MODEL TO PREDICT THE PARTICLE-SIZE DISTRIBUTION NUMBER DENSITY VERTICAL STRUCTURE AND OTHER PROPERTIES OF TROPOSPHERIC AEROSOLS. THE MODEL RESULTS WILL BE DIRECTLY COMPARED WITH GALILEO NIMS AND CASSINI VIMS AND CIRS SPECTRA TO CONSTRAIN CERTAIN CHEMICAL AND TRANSPORT PROPERTIES ACROSS THE PLANETS. THE GOALS OF THE PROPOSED WORK ARE (1) TO DETERMINE THE DOMINANT GAS-PHASE AND CONDENSABLE PRODUCTS FROM JOVIAN AND SATURNIAN TROPOSPHERIC PHOTOCHEMISTRY (2)TO QUANTIFY THE RELATIVE ROLES OF THE DIFFERENT PROCESSES THAT CONTROL THE OBSERVED VERTICAL AND MERIDIONAL DISTRIBUTIONS OF NH3 AND PH3 (3) TO DETERMINE THE ORIGIN AND COMPOSITION OF THE TROPOSPHERIC HAZE (4) TO MAP THE INFERRED VARIATIONS IN TROPOSPHERIC VERTICAL TRANSPORT PARAMETERS (E.G. VERTICAL WINDS AND/OR EDDY DIFFUSION COEFFICIENTS) ACROSS THE PLANETS FROM MODEL-DATA COMPARISONS AND (5) TO PREDICT THE SEASONAL VARIATIONS IN TROPOSPHERIC COMPOSITION ON SATURN. THE MODELS WILL SIGNIFICANTLY IMPROVE OUR UNDERSTANDING OF THE GENERAL PROPERTIES AND CHEMICAL-DYNAMICAL BEHAVIOR OF GAS-GIANT ATMOSPHERES AND WILL ENHANCE THE SCIENTIFIC RETURN FROM THE GALILEO AND CASSINI MISSIONS.