This study examines the underinvoicing of exports of copper concentrates and refined copper cathodes from Chile and Peru. A novel methodology, which takes the industry’s standard contractual terms and insurance and freight costs into account, is used to reveal the existence of below-market unit prices in copper trading. A sharp decline since 2011 in the extent of underinvoicing reflects improvements in the area of international taxation in the two countries; however, this progress represents no more than the first steps towards the establishment of a strong enough legal framework and institutional structure for the proper oversight of mining activity in its entirety. Given the complexity of evaluating and inspecting transactions of mining products, attention is drawn to the importance of strengthening tax and customs administrations.