The implied premium of plans by Deutsche Boerse AG (DB1.XE) to acquire NYSE Euronext (NYX) was eroded on Monday as speculation swirled about a counterbid for the operator of the New York Stock Exchange.

Unsourced media reports suggested CME Group Inc. (CME) could team with Nasdaq OMX Group Inc. (NDAQ) to bid for NYSE Euronext, splitting its derivatives and cash-equity operations.

Such a move would reprise efforts six years ago by Chicago-based CME to bolster its European expansion through acquiring the Liffe derivatives business now owned by NYSE Euronext. But Nasdaq would face major regulatory hurdles in securing approval to combine with the Big Board, despite intense competition in the U.S. cash-equities sector. Such a tie would create a monopoly in U.S. stock-market listings.

CME, Nasdaq and NYSE Euronext all declined to comment, but the speculation helped push up share prices across the sector.

Deutsche Boerse and NYSE Euronext had been seen closing a planned stock-swap by the end of the year, according to people familiar with the situation, and further details emerged about the governance of what would be the world's largest financial-exchange group.

The eight-member executive committee of the yet-to-be-named group would be drawn equally from each company, with four NYSE Euronext executives holding pivotal positions despite its shareholders' owning around 40% of the new entity.

Duncan Niederauer, chief executive of NYSE Euronext, has already been identified as CEO of the enlarged company. His deputy, Dominique Cerutti, would be the Paris-based head of global information technology and General Counsel John Halvey would retain that position, according to people familiar with the situation.

Larry Liebowitz would remain chief operating officer and oversee U.S. equities and global listings from New York, according to these people.

Deutsche Boerse chief executive Reto Francioni has been identified as chairman, while sources said his deputy, Andreas Preuss, would head the global derivatives operation from Frankfurt. Preuss oversees the company's Eurex derivatives arm.

A formal announcement was expected Tuesday following board meetings at both companies, while Deutsche Boerse is bringing forward its fourth-quarter-results announcement by a day to encompass the expected deal.

NYSE shares were recently up 2.5% at $39.26, having been lower for most of the session, while Deutsche Boerse was 0.5% lower at 61.33 euros ($83.08), trimming its indicative share of the combined market value to 61.4%.

The German group's stock price has been steady at around 62% of the combined group's market value since news of "advanced" talks between the two was leaked last week, indicating it would be paying a premium of around 5.7% for NYSE Euronext. The differential had halved during the session, and in late trading Deutsche Borse was seen paying a premium of $300 million.

   -By Jacob Bunge, William Launder and Aaron Lucchetti, 
   The Wall Street Journal; 312-750-4117; 
   jacob.bunge@dowjones.com

--Doug Cameron contributed to this article.
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