The prospect that governments of one or a few large countries, or trading blocs, would engage in international greenhouse gas emissions trading has led several policy analysts to express concerns that trade would be influenced by market power. The experiment reported here mimics a case where twelve countries, one of which is a large buyer (the mirror-image of a large seller), trade carbon emissions on an emissions exchange (a double-auction market) and where traders have quite accurate information about the underlying net demand. The findings deviate from those of the standard version of market power effects in that trade volumes and prices converge on competitive levels.Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/).