However, despite some initial efforts, reform in Mexican petroleum has faltered. Foreign participation in the exploitation of Mexico's petroleum resources has long been a particularly sensitive matter in Mexican politics.
Although, substantial early development of the Mexican petroleum industry was accomplished by British and U.S. petroleum companies {see Endnote 53}, nationalization of all foreign petroleum assets on March 18, 1938 abruptly ended foreign activity in Mexican Petroleum {see Endnote 54}. Foreign participation in oil and gas exploration, production, and refining is still proscribed by the Mexican constitution, which allows only Pemex to engage in these activities. So far, the only substantive reform measures include a restructuring of Pemex's operations (along with substantial reductions in employment), an attempt to sell several chemical units and other non-core operations, and an increased reliance on foreign drilling contractors. Pemex also has undertaken joint ventures abroad. Pemex's downstream operations have focused on reconfiguring and modernizing its refineries to both increase product output and address environmental concerns. Despite refinery upgrades, Pemex's refinery capacity is less than current product consumption, leading to increased product imports and further refinery construction. Pemex also replaced some of its domestic shortfall by gaining a 50-percent share in Shell Oil's Deer Park, Texas, refinery {see Endnote 55}.