E. J. Witzemann served as major professor for Cohen's Ph.D. work, and Professor Harold C. Bradley was on his examining committee. E. B. Fred, a member of the National Academy of Sciences, signed Cohen's thesis as dean of the graduate school. The thesis was entitled ''Studies in Ketogenesis,'' and it covered preparation of tissue slices and breis, the synthesis of several hydroxy and alpha-keto acids, the determination of acetoacetic and beta-hydroxy butyric acids, and the oxidation of a variety of fatty acids, amino acids, and keto and hydroxy acids. The studies were published in the Journal of Biological Chemistry (1937, 1938).
In June 1935 Cohen married Rubye H. Tepper, who had been a student in the University of Wisconsin Medical School. They had four children: Philip T., David B., Julie A., and Milton T., all of whom were students in the Madison, Wisconsin, school system.
After completing his Ph.D. and M.D. degrees, Phil Cohen was awarded a National Research Council Fellowship, and he spent 1938–39 in the laboratory of Hans A. Krebs in Sheffield, England, before returning to Yale University to complete the fellowship. The Krebs laboratory was a very lively research center, and Krebs and Cohen published a paper in Nature (1939) on glutamic acid as a hydrogen carrier in animal tissues. In 1939 they also had a paper in the Biochemical Journal on the metabolism of α-ketoglutaric acid in animal tissues. Thus, Cohen's many papers on nitrogen metabolism in animal tissues were launched with a distinguished investigator (Krebs later received the Nobel Prize) in prestigious journals.
Cohen (1939) also was sole author of two papers in the Biochemical Journal. The first detailed a method for the microdetermination of glutamic acid, which involved the conversion of glutamic acid to succinic acid with chloramine T plus acid hydrolysis, followed by the manometric determi-