The deal is potentially worth over EUR2.3 billion (US$3.3 billion) on the basis of Airbus' catalog prices.
ILFC, the world's leading commercial aircraft leasing company and Airbus' biggest customer, had originally ordered 16 aircraft based on a first design for the A350 that Airbus scrapped in 2006.
ILFC has signed up for the A350-800 and A350-900 versions of the aircraft, with the possibility of switching into the larger A350-1000 version.
The ILFC contract brings total orders for the A350 to 196 from nine customers.
Airbus is still in the process of deciding on suppliers for the aircraft, which will enter service with airlines in 2013.