THE FORMATION OF EXOPLANETARY SYSTEMS IS CONTINGENT ON THE AVAILABILITY OF RAW MATERIAL IN THE PARENT DISK ENVIRONMENT. THE TYPE OF PLANET FORMED (ROCKY OR GASEOUS) AT A PARTICULAR LOCATION IS EXPECTED TO DEPEND ON THE RELATIVE MASSES OF GAS AND DUST IN THE DISK. MOREOVER CURRENT THEORIES ARGUE FOR A DECREASE IN THE GAS/SOLIDS RATIO TO ENABLE THE FIRST CRITICAL STAGES OF PLANETESIMAL FORMATION. DUST EMISSION IS NOW KNOWN TO DECLINE WITH SYSTEM AGE. RECENT SURVEYS FOR STAR-FORMING REGIONS FURTHER INDICATE THAT THE DUST IN DISKS UNDERGOES EVOLUTION IN ACCORDANCE WITH WHAT IS EXPECTED THEORETICALLY. HOWEVER TRACKING THE GAS MASS HAS BEEN FAR MORE DIFFICULT WITH THE HITHERTO RELIABLE CO COMPONENT BEING FROZEN ON GRAINS AND PERHAPS SUBJECT TO CHEMICAL EFFECTS. WE PROPOSE TO USE NEW UPTO- DATE DISK CHEMICAL MODELS THAT INCLUDE ISOTOPE SELECTIVE PHOTO-DISSOCIATION AND GRAIN CHEMISTRY TO ESTIMATE THE MASS OF GAS IN DISKS. WE WILL ALSO RE-VISIT CONTINUUM-BASED ESTIMATES OF DUST MASS FROM EXTANT SURVEYS. FROM INDEPENDENT ASSESSMENTS OF THE TWO WE WILL COMPARE THE INFERRED GAS/SOLIDS MASS RATIO IN DISKS WITH DISK EVOLUTION AND PLANET FORMATION THEORIES. WE WILL THUS EVALUATE HOW THE MASS RESERVOIR IN PROTO-PLANETARY DISKS EVOLVES DURING PLANET FORMATION EPOCHS. THE PROPOSED RESEARCH IS DIRECTLY RELEVANT TO IMPROVING OUR UNDERSTANDING OF THE ORIGINS OF EXOPLANETARY SYSTEMS AND OF THE COMPOSITION OF EXOPLANETS (PREDOMINANTLY ROCKY ICY OR GASEOUS) AND HENCE ALIGNED WITH THE OBJECTIVES OF THE NASA EXOPLANETS RESEARCH PROGRAM.