Officers in Croydon are investigating incidents this week in which tried to set light to people sleeping rough.
Detectives believe others may have been targeted and are appealing for any further victims and any witnesses to come forward.
There were two incidents reported in the area of George Street and Surrey Street in the early hours of Monday, 10 June.
Two men were treated by the London Ambulance Service for minor burns. Sleeping bags and other property was set alight.
At around 5am on Wednesday, 12 June, a man and a woman were stopped by police on George Street in Croydon and arrested.
The 22-year-old man was later charged and is due to appear in court on Thursday, 13 June.
The 17-year-old girl was bailed until a date in July pending further enquiries.
Anyone with information concerning this incident is request to call police on 101 quoting reference 1018/10 June or Crimestoppers anonymously via 0800 555 111.