He became known as an authority on catalysis by acids and bases. He has the Brønsted catalysis equation named after him. He also came up with the highly used theory of the proton donor along with Lowry. Brønsted theorised that as a hydrogen atom (always found in an acid) is ionized once dissolved in water, it loses its electron and becomes a proton donor. The hydroxide ion, which occurs when an alkali is formed when a substance is dissolved in water is called a proton receiver. This leads to a neutralization reaction where the ions combine creating hydrogen hydroxide, otherwise known as water. The pH scale may be interpreted as "power of hydrogen", and the definition is based on the work of Brønsted and Lowry.
In World War II he opposed the Nazis, and in consequence he was elected to the Danishparliament in 1947, but could not take his seat because of illness. He died shortly after election.