Qatar Airways will finalise its agreement for a 35% stake in freight carrier Cargolux on 9 June.
The Doha-based carrier signed a memorandum of understanding to acquire the stake in May 2011.
A Cargolux spokeswoman said on 8 June that the deal would be "finalised tomorrow". She declined to disclose further details.
Qatar Airways chief executive Akbar Al Baker told ATI and Flightglobal in May that talks between the two carriers had begun six years ago.
Luxair holds a 52.1% stake in Cargolux, making it the largest shareholder. SAirLines (part of the defunct Swissair Group) sold its 33.7% stake in 2009 to three existing shareholders (including Luxair) and a new shareholder, the Luxembourg State.
The source of the Qatar Airways stake has not been disclosed.