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."

Lundy was found guilty in 2002 of killing Christine and seven-year-old Amber in 2000, and was jailed for life.
But the conviction was overturned in 2013 by Britain's Privy Council after new evidence emerged.
The jury at the High Court in Wellington unanimously returned a new guilty verdict on Wednesday, and Lundy was sentenced again to life in prison.
New Zealand media reported that Lundy stood open-mouthed in the dock as the verdict was delivered.
His brother, Craig, told reporters the ruling "brings some form of closure" for the family.
"For the past few months we have had to relive the moment we were told a despicable human being took the lives of our much cherished sister-in-law Christine, and our beautiful niece, Amber," the New Zealand Herald quoted him as saying.
Justice Simon France said time already served in prison would be taken into account in Lundy's sentence.
Christine and Amber Lundy were found dead at their home in Palmerston North in August 2000. They had been killed by multiple blows to the head from an axe or tomahawk.
Lundy was accused of staging a burglary to killed them for insurance money. Police believe Amber was killed as she tried to run away after witnessing her mother's murder.
Lundy, who was charged in February 2000, has always denied the murders, saying he was on a business trip in Wellington at the time and with a prostitute that evening.
Following his first conviction, a group of supporters compiled a lengthy dossier questioning the evidence presented in court, including DNA samples found on his shirt which the prosecution said were Mrs Lundy's brain tissue.
The appeal was lodged in Britain because New Zealand did not have a Supreme Court at the time.
The Privy Council ordered a retrial and Lundy released on bail in 2013.
Two people are being held on suspicion of murder after a man was found dead following a house fire in London.
[NEXT_CONCEPT]
A New Zealand court has found Mark Lundy guilty for a second time of murdering his wife and daughter.