Spokesperson Joseph Mabuza said four people died when a taxi collided with a truck that was u-turning on the freeway near Ermelo on Friday night.
Six people were seriously injured and taken to the Piet Retief Hospital.
Mabuza said the truck driver initially ran away after the accident, but later handed himself over to police. He was charged with culpable homicide and reckless driving.
On Saturday morning, eight people were killed when a taxi overturned after one of its tyres burst.
Five people were seriously injured and taken to Ermelo hospital.
Mpumalanga Community Safety, Security and Liaison MEC Vusi Shongwe sent his condolences to the families of those who were killed in the accidents.
Shongwe commended the police for charging the truck driver. He said it would ensure that drivers take road safety seriously.