South Wales Police is investigating criminal damage to a vacant property in Aberdare.
The fire in Brynhyfryd, Cwmaman, was reported to police at around 12.30am on Tuesday, April 6.
The blaze, which damaged the letter box, was extinguished by a neighbour using a bucket of water.
Neighbourhood officers have been carrying out house to house enquiries while scenes of crime officers are examining the scene.
Detective Inspector Phil Camm, from Cynon CID, said: “Thank to the quick thinking actions of local residents the fire was put out quickly and fortunately nobody was injured.
“However it is being treated as arson and would appeal for anyone with information to contact us.
“CCTV footage from the area is due to be checked but in the meantime I would ask anyone with witnessed suspicious behaviour in the area late on Monday night into the early hours of Tuesday to contact Aberdare CID on 01685 724284 or Crimestoppers anonymously on 0800 555 111.”
