A spokeswoman for Staffan de Mistura says the talks will officially start Wednesday but logistics and other issues mean that delegations are likely to arrive in Geneva over several days.
Spokeswoman Jessy Chahine specified the timetable Tuesday, amid media reports giving various dates for the start of the talks, AP reported.
The last attempt at peace talks collapsed within days in early February over a Russian-backed government offensive near the northern city of Aleppo. A US and Russian-brokered cease-fire that began late last month has significantly reduced the violence.
Chahine says a task force focused on humanitarian aid and ceasing hostilities would meet Wednesday.