A typical step in trade liberalization under the GATT is tariffication—the conversion of quantitative import restrictions to their ad valorem tariff equivalents. This paper shows that, if there is market power in the protected industry, tariffication may cause a global efficiency loss. In particular, in a small country, if the protected industry is a monopoly that is freely able to export but cannot profitably do so, then tariffication unambiguously imposes global efficiency costs. In a large country, the global efficiency effects are uncertain a priori. In both cases, however, tariffication unambiguously benefits the monopoly and lowers foreign welfare.