The paper shows how analyses assuming perfect competition can yield a distorted estimation of the expected effects of a trade liberalization when market imperfections exist. The analytical framework adopted is very simple and three extreme imperfect market structures are considered. In the first case, the exporting country maximizes its producer and consumer surplus by intervening in the world market. The second market imperfection considered is the existence of a private firm playing the role of "pure middleman" in the world market. Then the case of a producer-owned marketing board which is granted exclusive export authority is addressed. It is shown that under all three scenarios, if perfect competition is assumed when market imperfections exist, the impact of a tariff reduction on prices and volume traded is overestimated. A ranking of the size of such distortions in the three cases analyzed is provided. Finally, it is proved that when a private firm exerts monopoly and monopsony power in the world market, both the importing and the exporting countries may well be better off if, rather than making a move towards trade liberalization, the importing country "compensates" the exporting country by means of a direct transfer.