Three people have been arrested after a mother and child were allegedly sprayed with a substance after answering the door to their home in Fitzrovia.
Police and paramedics were called at about 5.20pm on Saturday to New Cavendish Street.
A female suspect had allegedly sprayed an irritant at the woman, aged in her 40s, and nine-year-old girl before fleeing, Scotland Yard said.
Both were taken to a central London hospital for treatment. Three people have since been arrested and bailed.
A woman, aged in her 30s, has been arrested on suspicion of GBH with intent.
A man aged in his 50s and a second woman aged in her 70s have been arrested on suspicion of conspiracy to commit GBH.
Officers are still investigating the circumstances of the incident, and are particularly keen to trace a man, wearing a distinctive orange jacket, who chased the female suspect in the Langham Street area immediately after the attack.
They also want to speak to anyone who was in the vicinity of the Yorkshire Grey pub on Langham Street who would have been aware of the altercation.
Anyone with information is asked to contact the Westminster Community Safety Unit on 020 7321 8033 or call Crimestoppers anonymously on 0800 555 111.
