"The adjudicatory chamber of the independent Ethics Committee ... has notified Mr Joseph S. Blatter and Mr Michel Platini of the grounds for the decisions passed in December 2015," the body said in a statement.
In December, it banned the Fifa president and vice president from football for eight years, saying they had abused their positions over a $2 million payment made to Platini in 2011 for work carried out between 1999 and 2002.
Blatter, who has headed Fifa since 1998, was also fined 50,000 francs ($50,000) while Platini, the head of Uefa, Europe's governing body, was fined 80,000 francs.
At the time of the verdict, which further heightened the crisis rocking football's scandal-plagued world body, the court insisted there was "no legal basis" for the payment that Blatter authorized for Platini in 2011.
The tribunal did not provide further details of the reasoning behind its decision.
Instead it stressed it had now "fulfilled its commitment to provide the grounds for the respective decisions to Mr Blatter and Mr Platini within the first half of January 2016."
"After receiving the grounds for the decisions, both officials may lodge an appeal with the Fifa Appeal Committee," the statement said.
If that appeal is rejected, the two men can appeal further to the Court of Arbitration for Sport (CAS), the highest tribunal in sports.
Vowed to fight
At the time of the verdict, both men angrily vowed to fight the bans, which started immediately.
The tribunal decision promises to end 79-year-old Blatter's four decades with Fifa in disgrace.
It also dealt a devastating blow to 60-year-old Platini's hopes of taking over as head of Fifa in an election on .
The Uefa president pulled out of the race earlier this week, saying the ban has made it impossible for him to put together a campaign to take on the sport's most powerful job.
He told French sports newspaper L'Equipe that he no longer had "the time nor the means to go to the voters, to meet people, to fight against the other candidates.
"In withdrawing, I am dedicating myself to my defence."
Fifa has since last May be rocked to its core by a cascade of corruption charges and arrests that culminated with the implication of the two long considered world football's most powerful men.
The US justice department has charged 39 individuals and two companies over
graft within world football going back decades, in a sweeping prosecution that has sparked an unprecedented crisis at Fifa.
A total of nine Fifa officials were arrested during two raids at the five-star Baur au Lac hotel in Zurich, on , by Swiss police acting on US warrants.