The October 20-26 reunion will be only the second such event in the last five years and there are concerns that North Korea may still cancel at the last minute -- as it has done in the past.
But Seoul's Unification Ministry said it was proceeding with preparations, with 35 people, mostly maintenance workers, driving through the eastern section of the heavily militarised border en route to the Mount Kumgang resort venue.
Seoul halted the tours in 2008 after a South Korean tourist was shot and killed after straying into a military area.
Unification Ministry spokeswoman Park Soo-Jin said the team that crossed into the North on Thursday was tasked with checking the resort's overall infrastructure -- including running water and electricity.
More From This Section
"Several dozen South Korean workers will travel in and out of the resort between now and October 14," she said.
Most died without having a chance to see or hear from their families on the other side of the border, across which all civilian communication is banned.
About 66,000 South Koreans -- many of them in their 80s or 90s -- are on the waiting list for an eventual reunion, but only a very limited number are chosen each time.
North and South are each selecting 100 people to attend next month's event, which is being held in line with an agreement reached last month aimed at defusing military tensions on the peninsula.
There has been widespread speculation that the North is planning a satellite rocket launch to mark the 70th anniversary of the ruling Workers' Party on October 10.
The North insists its space programme is purely scientific, but the United States, South Korea and their allies deem any such rocket launch to be a disguised ballistic missile test that violates UN resolutions.
If the North goes ahead regardless, it will trigger a spike in military tensions on the peninsula that is likely to count the family reunion among its first victims.