This paper studies the optimal design of commodity taxation in an open economy with rm mobility and labor market heterogeneity. We develop a two-country general equilibrium model featuring monopolistic competition, endogenous rm relocation, and a segmented labor market where skilled labor has exible wages and unskilled labor faces wage rigidity. We compare the welfare e ects of two tax principles: the destination principle (taxation at the point of consumption) and the origin principle (taxation at the point of production). Our main result establishes the existence of a threshold condition: if skilled labor accounts for more than half of production income, the origin principle yields higher welfare in non-cooperative tax settings; otherwise, the destination principle is superior. This nding provides a tractable rule based on observable labor market parameters and o ers new insights into international tax coordination. The model contributes to the literature by endogenizing labor income responses and rm mobility, highlighting novel channels of tax incidence and policy spillovers in integrated markets.