The Collaborative International Dictionary of English v.0.48:

Mitigatory \Mit"i*ga*to*ry\, a.
   Tending to mitigate or alleviate; mitigative.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mitigatory
    adj 1: moderating pain or sorrow by making it easier to bear
           [syn: alleviative, alleviatory, lenitive,
           mitigative, mitigatory, palliative]