Four women and one man - all in their 20s and 30s - were presumed to have inhaled the fumes by burning coal briquettes inside a parked car in Hwaseong, just south of Seoul on Wednesday, police official Song Ui-chan said.
Song said two of the five left suicide notes, though he declined to elaborate on their contents. An investigation is under way to find out whether the five met and planned a suicide pact over the Internet, he said.
Separately, three men were found dead hours later in Chuncheon, about 50 miles east of Seoul, police said without elaborating.
South Korea - which has the highest suicide rate among the 30 nations in the Organization for Economic Cooperation and Development - has had a string of high-profile suicides in recent years.
Choi Jin-sil, one of South Korea's most famous actresses committed suicide two years ago. In March, her younger brother, Choi Jin-young, also an actor, killed himself in Seoul.
Former President Roh Moo-hyun jumped to his death last May while embroiled in a widening corruption scandal, and the ex-chairman of South Korea's oldest conglomerate, Park Yong-oh of the Doosan Group, killed himself in November.
A 20-year-old top South Korean model who was a fashion week regular in New York, Milan and Paris was in her Paris apartment in November last year.
Yoon Dae-hyun, a psychiatrist at Seoul National University Hospital and an official of the Korean Association for Suicide prevention, said group suicide is not uncommon in South Korea, where the suicide rate has risen in recent years.
"The authorities are cracking down on Internet sites instigating suicide," Yoon said.