Amid investor concern that it is violating the spirit of accounting rules, Zions Bancorp has begun valuing its employee stock options using a controversial approach that relies on markets to set the price.

The decision by the Salt Lake City bank holding company may mark a critical new step in a battle over how to value employee stock options. After U.S. accounting standard setters in 2004 voted to require companies to begin treating stock options as an expense, companies began seeking Securities and Exchange Commission approval for new techniques that would lower those costs.