Eustace has scored 55 points for Gloucester during his career
Northampton Saints have signed lock forwards Adam Eustace and Mark Sorenson for the 2010/11 season.
Eustace, 31, has played over 200 times for Gloucester, while Sorenson, also 31, was Newcastle's players' player and supporters' player of 2007/08.
Saints director Jim Mallinder said: "We have a very young squad, and while this is exciting, we also need some experience around as well.
"Adam and Mark will both bring a record of consistently high standards."
He added: "They have proven themselves to be quality performers in the Guinness Premiership and are excellent additions to our squad ahead of next season.
"As well as being good players in their own right they will also be people who the younger players can learn from."
Eustace has played 26 times for Gloucester this season and, other than a year with Llanelli Scarlets, has spent his entire career with the club.
"This move to Northampton is a new challenge for me," Eustace said.
"It's a fresh start and a new lease of life. Every time I've been there, it's been a tough place to play.
"The fans and the facilities that the Saints have got are great, and the team that Jim and Dorian have put together are doing well. They've got a tough pack of forwards and backs who can score from anywhere.
"Northampton have also got the LV= Cup already and are competing in another two competitions. If it is the same next year, it'll be good to be a part of that as well."
Sorenson joined Newcastle in the autumn of 2006 from New Zealand province Bay of Plenty.
I've had three-and-a-half years at Newcastle but need something new to keep the rugby ambition alive
The New Zealander has made 25 appearances for the Falcons this season.
"I've got a burning ambition to play in the Heineken Cup," said Sorenson.
"Northampton are consistently making it into that competition now and are doing well this year. I'm also looking for a new challenge.
"I've had three-and-a-half years at Newcastle but need something new to keep the rugby ambition alive. I'll also be playing with a few ex-team-mates from up this way and I get along with them.
"When Northampton showed interest I spoke to them and it sounds like a professional set-up, which you can tell by the successful season they're having this year.
"There is progression and ambition and the club is going in the right direction."