Bolivia has taken full control of two oil refineries from the Brazilian state-owned energy company, Petrobras, after a compensation deal last month.
The head of the state energy company, YPFB, said Bolivia had taken back "what we never should have lost".
The $112m (£56m) buy-back is part of President Evo Morales' wider energy nationalisation, giving the state more control and a larger slice of profits.
Bolivia has South America's biggest natural gas reserves after Venezuela.
Constitution change
President Morales has pledged to reduce poverty by increasing state revenues from the energy sector, which was privatised in the 1990s.
The refineries, in the eastern city of Santa Cruz and central Cochabamba, were sold to Petrobras for $104m during that period.
After a long negotiation, Petrobras, the biggest foreign investor in Bolivia and leading natural gas producer, accepted $112m for the refineries, which process about 40,000 barrels of crude oil a day.
Speaking at a ceremony to mark the handover, the president said he intended to ask the popular assembly to write into the constitution a ban on any future privatisation of state companies.
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