His graduate studies were on the subject of carbon reduction during photosynthesis, working with Melvin Calvin in the Bio-Organic Chemistry Group of the Lawrence Radiation Laboratory at the University of California.
[2] He discovered, with Melvin Calvin and Andrew Benson, the Calvin-Benson-Bassham cycle.
He continued his work as Associate Director of this group.
Besides his work on the basic carbon reduction cycle of photosynthesis, Bassham conducted research on the biosynthetic paths leading from the cycle to the thermodynamics and kinetics of the carbon paths and the factors that control the flow of material and energy in this metabolic network.
He is coauthor (with Melvin Calvin) of "The Path of Carbon in Photosynthesis".