The rescue workers were returning to their office in the Pranburi district of Prachuab Khiri Khan province when the bus slammed into their vehicle from the opposite lane, police Lt. Col. Chuchart Namjat said.
All 10 people in the van, including the driver, were killed instantly, while three bus passengers were slightly bruised, he said.
The cause of the accident was being investigated, he said.
Prachuab Khiri Khan province is about 230 kilometers (145 miles) south of Bangkok.