The Mighty Avengers series was collected in 7 volumes:
Mighty Avengers Vol. 1: The Ultron Initiative (collects issues #1-6)
Mighty Avengers Vol. 2: Venom Bomb (collects issues #7-11)
Mighty Avengers Vol. 3: Secret Invasion Book 1 (collects issues #12-15)
Mighty Avengers Vol. 4: Secret Invasion Book 2 (collects issues #16-20)
Mighty Avengers: Earth's Mightiest (collects issues #21-26)
Mighty Avengers: The Unspoken (collects issues #27-31)
Siege: Mighty Avengers (collects issues #32-36)