<p>This paper examines the interaction between labor-market institutions and commodity tax-base design in an open economy with imperfect competition. We develop a two-country Cournot model in which a domestic firm bargains over wages with a monopoly labor union, while the foreign firm operates under a competitive labor market. Governments choose commodity tax rates under either the destination or the origin principle. We show that the welfare effects of unionization are systematically tax-base dependent. Under the destination principle, consumption-based taxation does not correct the production distortion induced by union wage bargaining, placing the unionized country at a disadvantage. Under the origin principle, production-based taxation directly interacts with union-induced cost distortions, yielding regime-specific welfare outcomes that depend on trade costs and the competitive wage. The analysis highlights tax-base design as a key institutional margin shaping strategic interaction and welfare in unionized oligopolies.</p>