Straw flew in on a chartered Airbus shortly before 2:00 amMonday).
German Foreign Minister Joschka Fischer had already arrived in the Iranian capital and French counterpart Dominique de Villepin was expected here before dawn.
Asked if he was confident of a deal on the nuclear standoff between Iran and the International Atomic Energy Agency (IAEA), Fischer said: "I hope so. We shall have talks and I hope the talks will be successful.
"We hope we will have productive and constructive discussions. These are very serious issues," Fischer told journalists ahead of talks between the foreign ministers and Iranian President Mohammad Khatami.
Iran indicated late Monday it was ready to sign an additional protocol to the nuclear Non-Proliferation Treaty in line with demands from the IAEA, which has set an October 31 deadline for Tehran's full cooperation.