Senior members of the Iranian negotiating team Abbas Araqchi and Majid Takht-e Ravanchi are holding talks with Helga Schmid and Wendy Sherman who respectively are deputy EU foreign policy chief and US Undersecretary of State.
The diplomats are to continue discussions on drafting the comprehensive nuclear agreement.
Deputies foreign ministers of Iran and the six world powers- the US, UK, France, China, Russia and Germany- are planned to hold bilateral meetings on Friday.
If today's talks advance in a proper way, the preliminary text of the comprehensive nuclear agreement will be written and presented to Iran's meeting with the Group 5+1 which is to be held on Friday, IRNA reports.
Vienna talks are aimed at lessen the parentheses and misunderstandings.
Coincident with the nuclear talks between the deputies foreign ministers, Iranian and European experts led respectively by Hamid Baeedinejad and Stephan Clement are working on the attachments.
If being written down, the text of the comprehensive agreement is to be in 20 pages while its attachments will include 50 pages.