A pressure group model where environmental and industry lobby groups offer political support in return for favorable pollution tax policies is used to explain and predict the equilibrium pollution tax in sectors protected by tariffs. The political economy effects of trade liberalization are investigated. The pollution tax is shown to decrease if the lobbying effort by the environmental lobby decreases more rapidly than by the industry lobby ceteris paribus. The level of political conflict falls with trade liberalization. Pollution may increase because of a reduction of the pollution tax, and tax revenues may fall simultaneously as pollution increases.