Different measures for carbon leakage prevention across Emissions Trading Systems (ETSs) may distort economic competition between firms. The same is true of competition between jurisdictions if decisions on the location of production plants are concerned. Free allocation of emission allowances responds to a logic of carbon costs compensation. Border carbon adjustments aim at levelling the playing field between domestic firms and their foreign competitors. Direct support to low-carbon innovation aims at enhancing the competitiveness of domestic firms. Any instrument for carbon leakage prevention could produce, depending on its own specific design, competitive distortions that are illegitimate under WTO law or other applicable trade regime. By inducing convergence of allowance prices, ETS linking reduces any internal competitive distortion due to differences in carbon prices. However, given pre-link differences in anti-leakage measures, price convergence can highlight or even exacerbate potential competitive distortions. In a linking context, output-based allocation may amplify or attenuate competitive distortions by interacting with post-link changes in allowance prices. Differences in anti-leakage measures do not preclude linking in a technical sense. However, with time, some harmonization may prove necessary for the political sustainability of the linkage. The actual significance of any competitive distortion always depends on the extent to which firms compete in a market.