Metropolitan Police were called at just after 3pm on Friday to concerns over occupants of a house in Staines Road, Bedfont, Hounslow.
Officers forced their way into the home and found the bodies of four people inside.
They are thought to be a man and woman in their 30s, an 11-year-old girl and a three year-old boy, who were all thought to be related.
Chief Superintendent Sean Wilson, local policing commander in west London, said: “Our investigation is at a very early stage, and based on our initial inquiries, we are not currently seeking anybody else in connection with the incident.
“I know the shock and distress that this terrible incident will cause among the community in Hounslow and beyond. I can assure local people that specialist officers are working to establish exactly what happened and I will provide further information as soon as I can.
“I am also aware that this incident will be particularly upsetting for children, and I ask people to please be responsible about what they post on social media, including not naming or speculating as to the names of those who’ve died.
“There will be additional police officers in the area across the weekend, I urge anyone with information or concerns to speak with those officers.”
