Neither he nor Knox had solid alibis for the night of Kercher's murder, Sollecito writes.
By the same law that protects Knox, Sollecito is also free to skip the retrial.
CNN: Analysis: Amanda Knox case is complicated, confusing mess
Neither Ms. Knox nor Mr. Sollecito is required to appear in Florence for the next trial.
Sollecito's father, Francesco, told CNN in a brief conversation the family was driving towards home.
Sollecito, 23 at the time, was Knox's boyfriend, studying computer science at another university in Perugia.
That may have been enough for the jury to set Knox and Sollecito free.
This is a good day for justice, and for Knox and Sollecito and their families.
Sollecito's father attended the high court hearing on Monday but he was not present for Tuesday's verdict.
CNN: Analysis: Amanda Knox case is complicated, confusing mess
They were intent on getting Sollecito to abandon Knox and accuse her of the murder, he wrote.
Sollecito describes his doubt about Knox's innocence at times, referring to the night the two were arrested.
There was no physical trace of them in the room where Kercher died, Knox and Sollecito's lawyers insisted.
The appeals court that acquitted Knox and Sollecito in 2011 criticized virtually the entire case mounted by prosecutors.
Sollecito's lead attorney, Giulia Bongiorno, a prominent parliamentarian, almost walked off the case because of the backroom deal.
Knox and Sollecito were initially convicted and given long prison sentences: 26 years for Knox, 25 for Sollecito.
Mr Sollecito's impassive face provided a stark contrast to Miss Knox who broke down in tears in the courtroom.
One of Mr Sollecito's lawyers, Giulia Bongiorno, said the delay in the court's decision was "very hard to interpret".
Sollecito was not studying at the same university as Knox, and is not being held in the same prison.
Lawyers for Knox and Sollecito had focused on discrediting DNA evidence that played a role in the 2009 conviction.
Another Rome law professor, Maurizio Bellacosa, said it is unlikely Knox and Sollecito would be sent back to prison.
The jury also freed Raffaele Sollecito, her former boyfriend who became her co-defendant.
Sollecito may well head for home if the jury decides in his favor.
Police arrested Knox and Raffaele Sollecito, who was her boyfriend at the time.
Guede chose to be tried separately from Knox and Sollecito because he feared a pact between the former lovers.
In 2011 Miss Knox and Mr Sollecito were acquitted on appeal, largely on the grounds that DNA evidence was flawed.
Miss Knox and Mr Sollecito had been facing 26-year and 25-year jail terms respectively following their initial convictions in 2009.
In the appeal, lawyers for Knox and Sollecito picked apart DNA evidence that played a role in the original conviction.
After nearly four years behind bars, Knox returned to her hometown of Seattle and Sollecito resumed his computer science studies.
After his release, Mr Sollecito said they were no longer in a relationship and Miss Knox had found a new boyfriend.
Knox and Sollecito were initially found guilty in connection with Kercher's murder, but were ultimately acquitted two years later on appeal.
CNN: Analysis: Amanda Knox case is complicated, confusing mess
Sollecito writes that at times, he was uncomfortable with Knox's "bizarre behavior, " which he says prosecutors used against both of them.
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