Serbian police are to investigate the 2008 privatization of state oil company NIS, now majority-owned by Russia's Gazprom Neft, the interior ministry said.
Gazprom Neft bought a 51 percent stake in the NIS monopoly for 400 million euros ($534 million) in 2008, under a Serbian government led by the current opposition Democratic Party.
Some politicians at the time said the sale grossly undervalued NIS. It coincided with the climax of a drive by Serbia's former southern province of Kosovo to secede, with Serbia relying on ally Russia's veto in the Security Council to block any UN endorsement of a declaration of independence.
"A special investigative team will examine all the facts and circumstances regarding the privatization of NIS," the ministry said in a statement late Monday.
The sale was part of a wider energy deal between Russia and Serbia, which included a commitment to build the South Stream pipeline through Serbia and construction of a gas storage facility since completed.
A spokeswoman for NIS said the sale "is a matter of agreement between the two countries."
"The matter should be resolved between the two countries," she added.
Serbia is in the middle of a dispute between the European Union, which it wants to join, and Russia's South Stream project, with the EU effectively blocking construction in Serbian neighbor Bulgaria.