The head-on crash occurred shortly before 10 p.m. Tuesday, on Haldimand Road 17 near the community of Dunnville.
Haldimand County OPP say their investigation led to one man being taken into custody.
That man later complained of injuries and was taken to a nearby hospital, after which he was transferred to a hospital in Hamilton.
The SIU is contacted whenever someone is killed or seriously injured while interacting with police.
Source: CTV News Kitchener