The children - two of three triplets - were found not breathing at a home in the Wildmill area of Bridgend on Saturday morning.
Emergency services were called at around 10.05am. There are no suspicious circumstances.
A police spokesman said: "The five-month-old children were conveyed to hospital where tragically they were later pronounced deceased.
"There are no suspicious circumstances surrounding the deaths which are being treated as a tragic accident."
It is believed one line of inquiry is whether sudden infant death syndrome (SIDS), also known as cot death, was involved.