70 ILCS 1870/2
All property of every kind owned by the Port District shall be exempt from taxation, however, a tax may be levied upon a lessee of the Port District by reason of the value of a leasehold estate separate and apart from the fee simple title, or upon such improvements as are constructed and owned by others than the Port District.
(from Ch. 19, par. 752)
(Source: P.A. 77-82.)