Indian coca farmer Evo Morales said he will not confiscate refineries or infrastructure owned by multinational corporations.
Instead, his government would renegotiate contracts so that the companies are partners, but not owners, in developing Bolivia's resources, he said.
"We will nationalise (Bolivia's) natural resources," Morales said at a news conference Tuesday in La Paz. "Many of these contracts signed by various governments are illegal and unconstitutional.
"It is not possible that our natural resources continue to be looted, exploited illegally, and as the lawyers say, these contracts are legally void and must be adjusted."
Bolivia's proven and potential reserves total 48.7 trillion cubic feet of natural gas, second only to Venezuela in South America, according to the US Department of Energy's Information Administration.
Morales said his government would open talks with governments and company executives, working to strengthen relations with state oil companies.
He has close relations with Hugo Chavez, the Venezuelan president, who is also trying to change the role of foreign oil companies in his country.
With 92% of polling stations officially counted, Morales had 54.1% of the vote in Sunday's election.
He needs a bare majority to win outright and avoid having congress choose between him and conservative rival Jorge Quiroga in mid-January.
Outgoing President Eduardo Rodriguez's administration said it was organising a transition team in anticipation of Morales' inauguration on 22 January.
On Monday, Morales said Brazilian oil company Petrobras must turn two refineries it owns in Bolivia back to Bolivian control.
"It is not possible that our natural resources continue to be looted, exploited illegally, and as the lawyers say, these contracts are legally void and must be adjusted"
Bolivia's new president
Morales announced that he had asked Luiz Inacio Lula da Silva, the Brazilian president, to return the refineries, which Petrobras purchased in the last decade.
Petrobras bought the two refineries from Bolivia's state-owned oil company in 1999 for roughly $100 million.
Nationalisation of hydrocarbons in Bolivia has become the key campaign issue for Morales.
But he has not laid out specific plans on how he will manage the nationalisation.
Bolivia in May had passed a new hydrocarbons law that raised oil and gas production taxes and royalties to 50%, and at least on paper made the state the sole owner of production.
But the interim government of President Rodriguez never decided how to put the stipulations into practice.
Morales didn't say when or under which terms his government would negotiate new contracts with the energy companies.
But he said that contracts with companies found to have been smuggling oil or gas, or dodging taxes in Bolivia, will simply be annulled.
The top investors in Bolivia are Petroleo Brasileiro SA, known as Petrobras, Spain's Repsol YPF, France's Total, British Gas and British Petroleum.
Foreign energy firms have invested $3.5 billion in Bolivia since 1996.
But after the passage of the new hydrocarbons law in May, and amid increasing calls for an outright nationalisation of the energy industry in Bolivia, they have mostly frozen any new investments for this year.