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Under the tutelage of Hermann Steudel, a nucleic acid chemical researcher, he found himself immersed in the investigation of what was then termed thymonucleic acid. This would later become know the as building block of all life on earth, DNA (deoxyribonucleic acid), with no little thanks to Feulgen. At the time thymonucleic acid was thought to have something to do with the transmission of genetic material and Dr. Steudel was using a technique called a biuret reaction to stain nucleic acids in order to discover their location inside or outside the cell walls. However the color was lost in the presence of water and hence unusable anywhere outside the lab. Feulgen, realizing the limitations of this technique, experimented with different compounds to overcome this problem. His first success was the combination of the stains Congo red and malachite green. By extracting the salts formed he avoided the water reaction and the stain remained. (Chieco & Derenzini)

Around 1914 Feulgen promoted the hypothesis that there were only four nucleotides forming any nucleic acid, which at the time was thought too few for the multiplicity of the gene pool. His own research later proved this correct. Feulgen's research was interrupted during this time with a stint as a military doctor in World War I. On March 13, 1916 in he married Frieda Brauns, who was also a medical student. It was around this time (1918) that he discovered that fushicn acid would react with nucleic acids turning them a bright fuchsia. While an interesting discovery he did not immediately recognize the significance of this staining technique. In 1919 he received his PhD and was offered a research fellowship at the Physiological Institute at Giessen. In honor for the work that he had done to date, Robert Feulgen was given the title of Extraordinary Professor for Physiological Chemistry in 1923. It was in 1924, with the help of his associate H. Rossenbeck, he published a paper on the subject of the fuschin acid staining of nucleic acids which was then termed then "Plasmalreaktion. Much later the name "Feulgen Reaction" was adopted by subsequent generations of scientist in honor of this simple yet remarkable technique. (Bennedum & Meusch) on December 10, 1927, in recognition of his services to science, Robert Feulgen was awarded the title of Personal Professor, and at the beginning of 1928 he was made Director of the Institute of Physiological Chemistry.

During his lifetime however, DNA was