Isis added more plastic to its mobile wallet yesterday when it signed up American Express to its contactless payment platform.

Isis, a three-way mobile commerce joint venture between AT&T, T-Mobile USA and Verizon Wireless, said American Express customers will be able to use its mobile wallet to make NFC payments after the service goes live later this year.

American Express first announced it would support Isis on its payment network last year, but this is the first time it has said it will allow its cards to be included in the mobile wallet. Its U.S. Consumer, Open Small Business and Serve cards will be available with Isis.

Visa, MasterCard and Discover will also allow Isis on their payment networks but have not said whether their cards will be included in the Isis mobile wallet. Chase, Capital One and Barclays added their cards to the wallet in February.

Isis had originally planned to build its own payments network, but it later scrapped the strategy and will now use systems set up by existing payment vendors.

Austin, Texas, and Salt Lake City will be the first launch markets. Isis has not listed a date for when the service will light up but has said it will be during the first half of this year.

Isis’ contactless payment system needs both NFC-based smartphones and compatible payment terminals. Top handset makers including HTC, Motorola Mobility and Samsung have agreed to come out with smartphones compatible with the mobile wallet service, and Isis has hired DeviceFidelity to develop a product that adds NFC to phones lacking the technology.

On the point-of-sale side, VeriFone, Ingencio, ViVOtech and Equinox Payments announced in March that they would include Isis' mobile payment system in current and future payment terminal lines.