The incident happened around 6am on Saturday at an address in Mayfield Drive, Armadale.
The 84-year-old man was alone in his home when two males came to his door claiming to be from the Gas Board and asking to check if he had a gas leak.
The victim allowed the men in to his home, where they searched the rooms before stealing his bank card, utilities cards and robbing him of a three figure sum of cash.
Both suspects then made off towards Mayfield Court.
Police say inquiries are ongoing to trace the man and urge anyone with information to come forward.
The first suspect is described as white, 30-40 years-old, 5ft 10ins tall with an Irish accent. He was wearing a Hi Viz vest, dark jacket and jeans, brown footwear, a black balaclava and gloves.
The second suspect is described as white, 30-40 years-old, 5ft 7ins talk with a stocky build and a local accents. He was wearing glasses and a Hi Viz vest, dark jacket and jeans, brown footwear, a black balaclava and gloves.
Detective Constable Jo McCall of Livingston CID said: “This was a frightening incident for the elderly victim within his home, which has naturally left him shaken.
“We are now appealing for witnesses who may have seen the suspects, or who recognise the descriptions to come forward.
“We’d also like to remind the public to be vigilant and ensure you ask for identification and if in any doubt do not let the persons in to your home until their identity can be verified by the agency or company.
“Always remember you are under no obligation to let a stranger into your home if you are unsure.”