A burglar tricked his way into a 93-year-old woman's house by claiming he was from the "electricity board".
The thief fitted a device to a radiator in the victim's kitchen and asked her to hold it so he could take a reading, but then walked off.
When the victim became suspicious she found him in the living room stealing £150 from her purse.
She challenged the thief but he left her home in Harwell with the money and the device at 1030 GMT on Tuesday.
The thief is described as about 5ft (1.5m) tall, aged in his mid 50s and was clean-shaven, with grey hair in a side parting that was swept back.
He was wearing a navy blue light-weight, waist-length jacket, black trousers and black shoes.
Anyone with information is asked to contact Thames Valley Police.