About half of Richards's scientific research concerned atomic weights, starting in 1886 with his graduate studies. On returning to Harvard in 1889, this was his first line of research, continuing up to his death.  According to Forbes, by 1932 the atomic weights of 55 elements had been studied by Richards and his students. Among the potential sources of error Richards uncovered in such determinations was the tendency of certain salts to occlude gases or foreign solutes on precipitation. As an example of the care Richards used in his work, Emsley reports that he carried out 15,000 recrystallizations of thulium bromate in order to obtain the pure element thulium for an atomic weight measurement.
Richards was the first to show, by chemical analysis, that an element could have different atomic weights. He was asked to analyze samples of naturally occurring lead and lead produced by radioactive decay.  His measurements showed that the two samples had different atomic weights, supporting the concepts of isotopes.
Although Richardss chemical determinations of atomic weights were highly significant for their time, they have largely been superseded.  Modern scientists use electronic instrumentation, such as mass spectrometers, to determine both the masses and the abundances of an elements isotopes.  From this information, an average atomic mass can be calculated, and compared to the values measured by Richards.  The modern methods are faster and more sensitive than those on which Richards had to rely, but not necessarily less expensive.
Other scientific work of Theodore Richards included investigations of the compressibilities of atoms, heats of solution and neutralization, and the electrochemistry of amalgams.  His investigation of electrochemical potentials at low temperatures was among the work that led, in the hands of others, to the Nernst heat theorem and the Third law of thermodynamics, although not without heated debate between Nernst and Richards.
Richards also is credited with the invention of the adiabatic calorimeter as well as the nephelometer, which was devised for his work on the atomic weight of strontium.