Low cost airline Ryanair was guaranteed more than £1m in a five year marketing agreement to fly into City of Derry Airport.
Ryanair paid the council £100 each time an aircraft landed
However, Derry City Council said only £300,000 of this came from ratepayers.
A journalist took the council to an information tribunal to reveal the terms of the deal struck in 1999.
Press Association journalist Brian Hutton fought a two year battle with the council to reveal the terms of the deal.
They were ultimately only disclosed after he took the council to the Information Commissioner and then an Information Tribunal.
He said under the deal, Ryanair was guaranteed £250,000 each year for five years, to promote its Londonderry to London route.
It was also given free landing, navigation and parking facilities and provided with security and other services.
In return, Ryanair paid the council £100 each time one of its aircraft landed or took off from City of Derry.
The council said it contributed £60,000 a year towards the marketing cost, with the rest of the money coming from Donegal County Council, the Northern Ireland Tourist Board and Failte Ireland.
The deal ended with the publication of new EU guidelines last year, and the council now gets a payment of £3 a head from the airline for each passenger it carries through City of Derry.
A Derry City Council spokesman said: "This was a commercial arrangement between a private airline and a public airport.
"If the information had been released at the date of the request in January 2005, the information would have had a significant adverse impact on the commercial interests of the airport and that of the local economy.
"It would have been of significant interest to competitor airports and airlines."