American Express said Thursday it was paying $150 million to acquire Accertify Inc., an Itasca-based firm that specializes in fighting online fraud.

Founded in 2007 by Jeff Liesendahl and other veterans of Orbitz, Accertify provides airlines and other online merchants with an extra layer of security to help them combat identity theft and credit-card fraud.

The firm’s customers include Delta, JetBlue and Southwest Airlines and Sony, 1-800 Flowers, Barnes & Noble, Urban Outfitters, Electronic Arts, StubHub, Tickets.com, Autotrader and Zipcar.

The transaction is expected to close during the fourth quarter of 2010. American Express will run Accertify as a wholly owned subsidiary and its hosted software application will become part of the portfolio of anti-fraud measures offered to companies via the  American Express Global Merchant Services organization.

Accertify will remain in the Chicago area and will continue to be run by Liesendahl, its chief executive. He’ll be reporting to Bill Glenn, who is the president of Global Merchant Services.