Michael Faherty, 76, was found lying face down near an open fire in his living room in Galway, Ireland.
Apart from his body, investigators could find no other damage in the house.
Dr Kieran McLoughlin, the West Galway coroner, said it was the first time in his 25 year career that he had returned a spontaneous combustion verdict, which is believed to be the first in Ireland.
Spontaneous combustion occurs when an object – in this case a human – bursts into flame from an internal chemical reaction, apparently without being ignited by an external heat source.
In most cases the victim is almost completely consumed, usually inside their home, while little to no damage is recorded to the property itself.
There have been hundreds of reported cases of death by spontaneous human combustion around the world over the last 300 years, although few have been properly assessed by experts.
The inquest on Thursday heard how investigators were baffled as to the cause of death, the Irish Independent reported.
Forensic experts found that a fire in the fireplace of the sitting room where the man's badly burned body was found had not been the cause of the blaze.
The court was told that no trace of an accelerant had been found and there had been nothing to suggest foul play. The coroner said he was satisfied nobody had entered or left the house that night.
The fire which killed Mr Faherty was confined to the sitting room. The only damage was to the body, which was totally burned, the ceiling above him and the floor underneath him.
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