The Collaborative International Dictionary of English v.0.48:

Isonitroso- \I`so*ni*tro"so-\ [Iso- + nitroso-.] (Chem.)
   A combining form (also used adjectively), signifying:
   Pertaining to, or designating, the characteristic,
   nitrogenous radical, NOH, called the isonitroso group.
   [1913 Webster]