Bahrain, Kuwait, Qatar and Saudi Arabia have signed an accord to create a monetary union.
The event took place on Sunday in the Saudi capital, Riyadh.
Oman announced in 2007 that it would not join, while the United Arab Emirates pulled out last month after the Gulf Co-operation Council
decided to base a future regional central bank in Riyadh.
The UAE wanted to host the central bank.
The date for monetary union date was originally set for 2010 but analysts consider that 2013 is a more realistic target.