The incident happened on Sept. 2 when the 43-year-old woman was talking on the phone to their son, RIA Novosti reported. The 45-year-old man allegedly grabbed the phone from her, smashed it, and then went to bed.
"The woman, outraged by her ex-husband's behavior, ran into the kitchen, grabbed a cup of spirit and a lighter, poured it [the spirit] over him and set fire to him," the regional Investigative Committee said in a statement.
The couple divorced more than two years ago, but were still living together, investigators said.
"After he caught fire, the woman immediately began to put out the flames, and then called emergency medical services and the police," the statement said.
The man died of severe burns on September 10.
Investigators said the woman is under investigation on suspicion of causing grievous bodily harm resulting in death by carelessness. She faces up to 15 years in prison if she is convicted on those charges.