In 1900 he became a demonstrator in chemistry at McGill University in Montreal, Quebec, Canada, where he worked with Ernest Rutherford on radioactivity. He and Rutherford realized that the anomalous behavior of radioactive elements was due to the fact that they decayed into other elements. This decay also produded alpha, beta, and gamma radiation.
From 1904 to 1914, he was a lecturer at the University of Glasgow and while there he showed that uranium decays to radium. It was here also that he showed that a radioactive element may have more than one atomic weight though the chemical properties are identical; this led to the concept of an isotope. Soddy later showed that non-radioactive elements also could have multiple isotopes. In addition he showed that an atom moves lower in atomic number by two places on alpga emission, higher by one place on beta emission. This was a fundamental step toward understanding the relationships among families of radioactive elements.
He died in Brighton, England.