In 1866, Nilson became an associate professor of chemistry at Uppsala, with the responsibilities of chief assistant and demonstrator of chemistry for Svanberg's laboratory. He was a calm and patient teacher, relying more on demonstrations than on lectures. He insisted that after the work of the day was completed, it was to be left behind, and not discussed during leisure. His earliest papers mainly concerned sulfides, arsenical sulfosalt minerals, and selenous acid, extending the work of Berzelius.