The panel are considering verdicts against Benjamin Field, 28, and Martyn Smith, 32, who are accused of killing Peter Farquhar, 69, and conspiring to murder Ann Moore-Martin, 83.
On their fifth day of deliberations at Oxford Crown Court the jury of six men and six women sent a note to the trial judge, Mr Justice Sweeney, complaining about the heat in their room.
They said they had the windows open and were using a fan but the "extreme heat" meant they were struggling to concentrate.
Calling the jury into the court room, Mr Justice Sweeney said that in the circumstances he was going to send them home early and resume on Wednesday morning.
He told them: "It is clearly not appropriate for you to continue to deliberate in the conditions you describe and therefore I am inviting you to stop deliberating for the day."
Others cases have been called off this week at Oxford Crown Court due to the broken air conditioning system not being repaired.
Temperatures have continued to rise across Britain with forecasters predicting temperatures of up to 37C (96.8F) before the end of the week.
If achieved, it would surpass the current record for a day in July - 36.7C (98.1F) at Heathrow Airport in 2015.
Field, a Baptist minister's son, has admitted fraudulently being in relationships with Mr Farquhar and his neighbour Miss Moore-Martin as part of a plot to get them to change their wills.
Prosecutors allege Field and Smith targeted Miss Moore-Martin a few months after allegedly murdering Mr Farquhar in the village of Maids Moreton, Buckinghamshire.
Mr Farquhar, who was torn about his sexuality because of his religion, died in October 2015, while retired teacher Miss Moore-Martin passed away in May 2017 from natural causes.
Field has admitted defrauding Miss Moore-Martin of £4,000 to buy a car but denies recruiting his younger brother Tom, 24, to con her out of £27,000 by claiming it was for a dialysis machine.
Field and Smith deny charges of murder, conspiracy to murder and possession of an article for the use in fraud.
Field, of Wellingborough Road, Olney, Buckinghamshire, also denies an alternative charge of attempted murder. But he has admitted four charges of fraud and two of burglary.
In addition Smith, of Penhalvean, Redruth, Cornwall, denies two charges of fraud and one of burglary.
Cambridge University graduate Tom Field, of Wellingborough Road, Olney, Buckinghamshire, denies a single charge of fraud.
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