Ontario Provincial Police say officers and emergency crews responded to a collision on Highway 10 late Friday, just before midnight.
OPP say the driver and passenger of one vehicle were pronounced dead at the scene and two occupants of a second vehicle were taken to a Toronto-area trauma centre with serious injuries.
Provincial police are asking anyone who witnessed the collision or with dashcam footage to contact Caledon OPP or anonymously contact Peel Crime Stoppers.