George Everitt, 78, and his wife Eileen, 74, from Boston, died in Chiang Mai on 19 February.
A coroner’s court in Spalding recorded an open verdict into their deaths.
The Thai authorities said the couple died of heart attacks, but the coroner said there was insufficient evidence to determine how they died.
Coroner Maureen Taylor said: “It is apparent the reason for their deaths may never be known.
“What started as an enjoyable holiday ended in tragedy and my sympathy is with the family who may never find the answers to their questions.”
‘Unbelievable circumstances’
Andrew Taylor, the Everitts’ son-in-law, said the verdict was frustrating.
“It has been nine months of unbelievable circumstances and you would like to get some sort of closure.
“We still don’t really know what caused it.”
There were two other deaths at the Downtown Inn in Chaing Mai several weeks before the Everitts died.
Mrs Everitt was found lying on the bed, while Mr Everitt was in a sitting position on the floor with his face falling forward on to the bed.
Home Office pathologist Guy Rutty told the inquest that an “atmospheric intoxicant” could have been to blame, but tests to find out what that was were inconclusive.
Mr Rutty said because the bodies were not sent to England for two months, it was difficult to carry out a second set of post-mortem tests due to the condition of the remains.
He said that simple carbon monoxide poisoning tests had not been carried out.
Report by : BBC
Seven Mystery Deaths In Chiang Mai Since January
Seven people have died in mysterious circumstances in Chiang Mai, northern Thailand since January this year, with authorities claiming that victims have been poisoned by “toxic seaweed”.
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There's no way they died from inhalation of any pesticide.