Fajans worked with Henry G. Moseley at the laboratory of Ernest Rutherford researching properties of the radioactive rows of the periodic table. He identified the half-lives of the uranium-actinium row and thorium nuclides. 
He discovered the phenomenon of the electrochemical branching of the radioactive rows. Afterwards Fajans worked on the electrochemical properties of elements as a result of the radioactive changes, and he formulated the law of the radioactive shifts which was later named the radioactive displacement law of Fajans and Soddy (Frederick Soddy received the Nobel Prize in chemistry in 1921 for his isotopic research). In 1913, together with Oswald Helmuth Göhring, he discovered the first radionuclide of a new element, which was later named protactinium. Fajans and Otto Hahn discovered the formula that defined the conditions of precipitation and absorption of radioactive substances. It is very significant for separation and purification of radioactive substances found in the smallest number.
In 1919, Fajans began researching the structure of crystals by thermochemical and refractometric methods. The co-relation of Born, Fajans and Haber is a basic thermochemical rule. On the basis of his research data Fajans formulated the essential relationships concerning chemical bond strength and deformation of ions and particles, such as heat of ion hydration, refractive index and the heat of sublimation. In 1923 he formulated Fajans' rules of inorganic chemistry, which are used to predict whether a chemical bond will be covalent or ionic.
In the United States he researched nuclear reactions using a cyclotron and discovered a radioactive lead isotope with Voigt, and a new rhenium isotope with Sullivan. He developed the quantum theory which explained chemical bonding through electrostatic impacts between quanta and nuclear cores. He was a member of the Polish Institute of Arts and Sciences in America and of many societies and academies.