The Sun also announced that they have signed Kristen Rasmussen from the Phoenix Mercury to a three-year contract.
A five-time All-Star, the 6-foot-2 McWilliams-Franklin has averaged 12.4 points and 7.5 rebounds during her eight years career. She had been with the franchise since its inception as the Orlando Miracle. She was the 32nd pick in the 1999 draft by Orlando, after having played three years in the now-defunct American Basketball League.
The 6-foot-5 De Souza is averaging 6.4 points, 3.3 rebounds and about 13 minutes for Ros Casares in the FIBA Euroleague. The Brazilian was the Spanish League's player of the year last season and led Universitat de Barcelona to the finals. She played for Los Angeles in 2002.
Sun coach Mike Thibault said the trade would be good for McWilliams-Franklin's family life because one of her daughters will be attending school in Arizona. He would not disclose other details of their conversations.
"This is a tough day for us because she's been a huge part of our success," Thibault said.
The Sparks announced Wednesday that they signed McWilliams-Franklin to a multiyear contract.
The Sun had the best regular season record (26-8) in the WNBA last season, but a championship again eluded them. They lost in the deciding game of the Eastern Conference finals to the rival Detroit Shock after having won the conference title the previous two years but coming up short in the league finals.
After the team was ousted from the playoffs, Thibault said there would be changes.