In theory, competitive emission permit markets minimise total abatement cost for any emission ceiling. Permit markets are often imperfectly competitive, however, if they are thin and dominated by large firms. The dominant firm(s) could exercise market power and increase other firms’ costs of pollution control, while reducing their own emission control costs. This paper reports a testbed laboratory experiment to examine whether a dominant firm can exercise market power in a permit market organised using the double auction trading institution. Our parameters approximate the abatement costs of sources in a proposed tradable emissions market for the reduction of nitrogen in the Port Phillip Watershed in Victoria, Australia. We vary across treatments the initial (pre-trade) allocation of permits to sources, so that in one treatment the seller of permits is a monopolist and in another treatment the selling side of the market is duopolistic. We also vary the information that subjects have about the number and abatement costs of their competitors. We find that prices and seller profits are higher and efficiency is lower on average in the monopoly sessions compared to the duopoly sessions, but the differences are not substantial and are not statistically significant due to pronounced variation across sessions. Moreover, prices, profits and transaction volumes are usually much closer to the competitive equilibrium than the monopoly equilibrium.