Yushchenko, who was an opposition leader at the time, fell severely ill during the fiercely contested 2004 presidential election campaign after having dinner with top security officials Ihor Smeshko and Volodymyr Satsyuk.
The illness left his face pockmarked and discolored and he was later diagnosed as having suffered massive dioxin poisoning.
No arrests have been made and the probe is still under way. But many observers point the finger at Russia — both because Yushchenko was running against a Kremlin-backed candidate and because Russia is one of four countries that produces the specific formula of dioxin used to poison him.
On Monday, Yushchenko annulled a January 2004 decree issued by his predecessor Leonid Kuchma which elevated Smeshko, then Ukraine's Security Service chief, to the rank of an extraordinary and plenipotentiary ambassador.
He also canceled Kuchma's August 2004 decree which gave Satsyuk, Smeshko's deputy at the time, a general's rank, dismissing both decrees as "groundless," according to the presidential Web site.
Yushchenko has complained that Russia was hampering the investigation by refusing to provide dioxin samples and hand over key suspects.
Ukrainian authorities have not named any suspects, but Yushchenko has said several of them are hiding out in Russia.
The Kremlin backed Yushchenko's rival, Viktor Yanukovych, in the 2004 presidential election, which deepened rifts between Moscow and the West.
Yanukovych was initially declared the winner. Massive street protests — dubbed the Orange Revolution — broke out, and the Supreme Court threw out the results on grounds of fraud. Yushchenko won a court-ordered repeat vote.
Yushchenko has hinted that he knows those responsible for the poisoning. While refraining from naming the alleged culprits until the investigation is over, he has intimated that the poisoning could have been masterminded from outside the country.
Source: International Herald Tribune