Isis mobile wallet adds AmEx cards in lead-up to launch

Isis, the mobile payment joint venture of AT&T (s t), Verizon (s vz)(s vod) and T-Mobile, has added American Express (s axp) cards to its wallet, which is set to debut this summer in Austin and Salt Lake City. The news helps build more momentum for the Isis payment platform, which now has a broad array of payment cards that will work inside the mobile wallet.

In February, Chase, Capital One(s cof) and Barclaycard announced they will place their credit, debit and pre-paid cards on the Isis wallet. That trio already gives access to about 50 percent of all consumers in the U.S. for Isis. Now, with American Express enabling its U.S. consumer, Open Small Business and Serve cards on the Isis platform, it should expand Isis’ reach even more. These types of deals are important for a mobile payment platform, because it means that more consumers will be able to use the wallet immediately when it becomes available.

That’s one of the things that has held Google Wallet (s goog) back. It launched with just Citibank (s c) as a partner, limiting the number of consumers who could take full advantage of the system.

Isis officials have told me in the past that this has been part of the reason it’s been slower to launch. They’ve been trying to ensure that when Isis gets going, it will have many familiar banks and retailers on the platform so consumers will feel comfortable in adopting it. That was echoed by Isis’ CEO Michael Abbott in a statement today.

“The relationship with American Express underscores our commitment to ensuring a flexible and inclusive platform that offers consumers choice in mobile payments while delivering new opportunities for merchants to connect with consumers,” said Michael Abbott, CEO, Isis. “Today’s announcement is testament to the vision and commitment of American Express to make mobile commerce a real and positive experience for their customers.”

Isis keeps adding critical pieces to its payment system. I’m still curious as to how many retailers will be on board at launch. Isis still has to get over the reality that most consumers and merchants are not equipped with the near-field communication (NFC) hardware to take advantage of its mobile wallet. But the fact that Isis is lining up this kind of broad support from financial institutions suggests it’s got a promising start ahead.