Thames Valley Police is appealing for witnesses following a traffic collision on the A418 between Aylesbury and Stone.
At around 5.44am on Wednesday (3/2), a collision occurred between a bus and a VW caddy van.
The driver of the van was left with serious injuries and was taken to hospital where he remains.
The bus driver received minor injuries.
Investigating officer PC Raymond Macnaught, of the Roads Policing Unit, said: “We are keen to speak to anyone travelling on the A418 around the time of the incident who may have seen the collision or the moments leading up to it.
“We would also ask those driving in the area at the time to check their dash cam footage in case they captured something that could be of use to our investigation.”
Anyone with information can report it online or via 101 with reference 43210046171