A well treatment fluid and method uses an organic peroxide. An activator embodiment can lower an effective concentration of the peroxide to break the fluid. A weight ratio of activator:organic peroxide can be at least about 1:20 in one well treatment fluid embodiment. A peroxyester breaker embodiment can be used in a well treatment fluid and method. A breaker delay agent embodiment can control the polymer break window in a well treatment fluid and method using an organic peroxide. A breaker package embodiment used in a well treatment fluid and method can include an organic peroxide and an amine breaker delay agent having the formula RR1NR2 wherein R, R1 and R2 are independently selected from hydrogen, alkyl, hydroxyalkyl, and combinations thereof.