Perchloric acid has the formula HClO₄ and is a colourless liquid soluble in water. It is a strong acid like sulphuric or nitric acid. M.p.-17°C, B.p.181°C, r.d.1.664. It is the strongest known acid.

This acid is very dangerous and can explode when concentrated. Its colour turns yellow before it explodes. Perchloric acid is an extremely powerful oxidizer and reacts explosively with organic compounds. As a result, it is often diluted with water.

An M/10 solution in glacial acetic acid is used as an analytical reagent. Titration of weak bases is made easier if the usual medium, water, is replaced by glacial acetic acid. Glacial acetic acid is a much weaker base than water, so the base being titrated appears to be stronger. As a counterpart, the strength of acids is reduced. This shows up the difference in strengths among the strong acids. Perchloric acid, being the strongest of all readily available acids, is therefore the acid of choice.

The diluted acid can be made by distillation of a solution of sodium perchlorate mixed with sulphuric acid. It also can be made by mixing nitric acid with ammonium perchlorate. The reaction gives nitrous oxide and perchloric acid. An alternative way is to react barium perchlorate with dilute sulphuric acid, which precipitates the barium sulphate, leaving only perchloric acid in solution.

The salts of perchloric acid are powerful oxidizers that are often used in explosive compositions. However, it is strange that perchlorates are even weaker oxidizing agents than chlorates.

It forms an azeotrope with water that boils at 203°C (perchloric acid 28.4% with water 71.6%)