A senior United States official is due to take part for the first time in international talks with Iran over its controversial nuclear programme.
William Burns is joining envoys from the European Union and permanent members of the United Nations security council.
They are expected to discuss with Iran's top nuclear negotiator incentives for Iran to suspend uranium enrichment.
Mr Burns' attendance is being seen as a major shift in US policy. The US and Iran have had no diplomatic relations since the 1979 Iranian Revolution and the taking of hostages at the US embassy in Tehran.
Since 2002 President George Bush's administration have insisted that no face-to-face talks would be held unless Iran suspended the enrichment of uranium, which it says could be used to produce nuclear weapons.
But Iran says its nuclear activities are only for civilian purposes.
US officials say Mr Burns' presence will be a "one-time event" and that he is in Geneva not to negotiate but to listen.
With just six months left of his presidency, analysts say Mr Bush is taking a more pragmatic approach.
US Secretary of State Condoleezza Rice seemed to sum up the new conciliatory mood saying that Iran was "a difficult and dangerous state" before adding: "We have been very clear that any country can change course."