Police were called by the landlord to the Bridge pub in Priestwood at 9.49pm because of concerns about the 34-year-old man.
Some time after the police arrived the man became unwell and an ambulance was called.
It is understood the man died at around 11.30pm in an ambulance at the scene. His next of kin have been advised and a post mortem took place on Monday, January 27, the result of which was inconclusive.
Assistant commissioner for the IPCC, Guido Liguori, said: "We will be seeking to establish the circumstances surrounding the interaction between Thames Valley police officers and this man before his death.
"I would like to offer my condolences to his family at this very difficult time.”