The victims were 42-year-old Lisa Lewis and 43-year-old Theddis Provall. They were pronounced dead at 10:40 a.m. at their home at 226 W. 111th Pl.
Authorities were notified when an adult daughter stopped by to check on the couple, and found them in their bed about 9:30 a.m. High levels of carbon monoxide were found in the home, according to the medical examiner's office.
A space heater that was being used in the home is suspected of causing the dangerous gas levels.
The residents were using a supplemental heater, according to Calumet District police Capt. James O'Donnell. Foul play is not suspected in the deaths, he said.
"She was a housekeeper for a nursing home, very outgoing, very loving. Her and Theddis, they've been together for over 25 years. They were lovebirds. It's just so, so sad," said Sheila Lewis, sister.
Fire Media Affairs spokesman Will Knight said fire crews responded to the scene because police requested them there, but no one was taken to a hospital.
Knight could not confirm the carbon monoxide information.
The medical examiner's office will conduct autopsies on the victims on Saturday.
Sun-Times Media Wire contributed to this report.