Six potential bidders from a total of seven consortia have submitted pre-qualification documents bidding for Madrid Barajas and Barcelona El Prat airports.
The Spanish government is selling 90.05% stakes in both of the country's main airports.
Five consortia are bidding for both airports, while a group led by Grupo San José is bidding solely for Madrid and a group led by Abertis Airports is bidding only for Barcelona.
The five groups bidding for both airports are led by BAA owner Ferrovial, India's GMR Infrastructure, Changi Airports International, Aéroports de Paris and Fraport.
Fraport is bidding in co-operation with Spanish construction firm Acciona, said a spokesman for the German airports group.
He added that yesterday's submission of pre-qualification documents was the first step in the privatisation process. If qualification is accepted, the next stage will be to prepare the bid.
Aena Airports, the new holding company established to move Spain's airports towards privatisation, said it will announce in the week beginning 12 September which of the bidders have met the demands required to pass through to the next stage.
The management contracts for Madrid and Barcelona are for 20 years and can be extended by a further five years. The Spanish government said in July that it expects the sale of Madrid Barajas to raise €3.7 billion ($5.2 billion) and the sale of Barcelona El Prat to raise €1.6 billion.
The successful bidders will have to pay an annual fee worth 20% of revenue for the airports.
Source: Air Transport Intelligence news