This low vaulted room off the East Cloister is part of the Undercroft that was built about 1070 but was walled off from the rest of the room sometime in the 12th century. The chamber was probably made into a treasury in the 13th century and may have been used as a sacristy when Henry III was rebuilding the main Abbey. This would explain the presence of the altar, recently dedicated to St Dunstan. The medieval tiled floor has designs similar to those on the much finer floor in the Chapter House, and show mainly heraldic subjects. In the time of Edward I this Chamber and the crypt of the Chapter House were assigned to the "Royal Wardrobe", a department of State. In 1303, when the King was away in Scotland, the Wardrobe treasury was burgled and money and plate stolen. Although the Abbot of Westminster and the monks were suspected of complicity and sent to the Tower of London, they were later released and one Richard de Podlicote and associates hanged for the crime. The present double oak security doors at the Chamber entrance were provided after this event and the room was then used to house some valuables of the Exchequer. There are two large rectangular chests in the Chamber dating from the 13th and 14th centuries which were evidently made inside the room.

But it was best known as the home of the wooden boxes, called Pyxes, where a sample of the coinage of the realm was kept to await the "Trial of the Pyx". This was a public demonstration to show that the coinage was pure and samples of coins were "tried" by being melted down and the silver content measured. The Trial itself was never held in the Chamber but in the Palace of Westminster. It still takes place today in Goldsmiths' Hall in the City of London.