Montreal police are investigating.
The fire started just before 5 a.m. in a duplex on Desmarteau street near Hochelaga street.
It appears a man and a woman were having a domestic dispute leading up to the fire, according to Montreal Fire Department Operations Chief Ian Ritchie.
It's not clear exactly how the fire started, but Montreal police Const. Manuel Couture said the 31-year-old man had been handling a flammable liquid.
Both the man and woman were taken to hospital to be treated for burns.
Couture said three other people living on the upper level of the duplex were treated for smoke inhalation, but they were not seriously injured.
The 31-year-old man was known to police.