American Express is having a very rewarding experience in the Australian cards market.In the half decade since RBA rules shifted the ground for everyone in payments American Express has lifted its share from 12 per cent in 2002 to 20 per cent in July 2010.Major banks are helping American Express build this windfall gain, with heavily promoted companion cards that earn the best rewards in the credit card market.In the original 1990s boom in credit cards with half decent rewards all banks, and Amex, and Diners Club, shared in the spoils (and following a lead set by ANZ).The RBA saw this as a misallocation of resources encouraged by inflated fee flows as banks either side of a payments machine paid each other for their services.MasterCard and Visa and their bank backers saw their most profitable product ruined as the RBA patrolled fees. Critics noted that American Express and also Diners Club were likely winners from these regulations.This is proving to be half right since the Citi-owned Diners Club near-immolated following the collapse of Ansett, leaving the field wide open to its rival.The American cards giant is earning above average yields from a rising market share, with a 16 per cent market share of transactions by number and the share by value at 20 per cent.Amex also earns higher yields from interest income.Mike Ebstein, principal of MWE Consulting wrote in his monthly report this week that "since 2003, with the exception of a reasonably stable period between 2005 and 2008, there has been a considerable gain by the three party schemes."There is no evidence to hand that the current gains being achieved by them are beginning to slow."