A 27-year-old man and a woman, 32, were detained after the 60-year-old victim's body was found at the Forest Gate house, early on Christmas Day.
Four people escaped from the house on Field Road before firefighters arrived just before 04:45 GMT.
A post-mortem test showed the victim had died from burns and the inhalation of fumes, the Met Police said.
Fire crews found his body on the ground-floor of the two-storey house.
Police believe "the fire was started deliberately" and say they believe they know who the victim was, but formal identification has not yet been made.
Twenty one firefighters and four engines tackled the blaze, which was brought under control after about two-and-a-half hours.
Det Ch Insp Steve McCabe said: "I need to hear from anyone who saw anything suspicious in Field Road and the surrounding area in the early hours of Christmas Day."
Two people are being held on suspicion of murder after a man was found dead following a house fire in London.