Iran's Foreign Minister Kamal Kharrazi has promised "total transparency" over the country's controversial nuclear programme.
The ministers are hoping for a breakthrough
Mr Kharrazi made the pledge during talks with his counterparts from Britain, France and Germany - who are visiting Tehran in the latest attempt to defuse the crisis.
The European ministers were expected to offer to help Iran have access to technology for peaceful nuclear energy production if it meets International Atomic Energy Agency (IAEA) demands for tougher inspections of its facilities.
But Germany's Joschka Fischer said it was a question of trust and transparency and warned of a "serious problem" if Tuesday's talks did not succeed.
The Vienna-based IAEA has given Iran until the 31 October to provide evidence that it is not trying to build nuclear weapons.
The BBC's Tehran correspondent Jim Muir says the three European ministers clearly would not risk such a high-profile visit unless a large measure of understanding had been reached in advance.
Iran insists that its nuclear programme - which includes uranium-enrichment activities - is designed to meet its energy needs only.
Quid pro quo?
Iranian President Mohammad Khatami is scheduled to meet Mr Fischer and his British and French counterparts - Jack Straw and Dominique de Villepin - later on Tuesday.
After meeting the ministers, Mr Kharrazi said his country was ready for "total transparency because we are not pursuing an illegal programme".
But, he insisted, "what's legitimate for us should be respected as well as our dignity and security" - a reference to Iran's stated ambition of generating its own nuclear power.
And Mr Fischer spoke of "a crucial moment in the international situation".
"It is a question of trust and transparency. If we can agree today it would be a step forward, if not we have a very serious problem," Mr Fischer told reporters.
The European ministers are also due to hold talks with the head of the Supreme National Security Council, Hassan Rohani, prior to meeting the president.
Our correspondent says the meeting with Mr Rohani is likely to be the crucial one.
Behind the scenes
The European initiative is part of a larger package being spearheaded by the IAEA, which has called on Iran to accept tougher UN inspections by signing an additional protocol to the Non-Proliferation Treaty (NPT).
According to diplomats, France, Germany and the UK have been engaged in a secretive effort to convince Iran to sign the protocol - which allows for snap inspections of nuclear facilities by the IAEA.
Tehran denies it has a nuclear weapons programme
The agency is also seeking clarification on traces of highly enriched uranium found in samples taken by its inspectors at an Iranian nuclear facility earlier this year.
Iran insists the traces were the result of contamination on imported equipment.
The three EU countries are believed to have offered technical assistance to Iran in exchange for its co-operation.
However, diplomats have said that not all outstanding issues had been resolved ahead of the talks.