Parameters of the modified UNIFAC model (Weidlich and Gmehling, 1987; Gmehling et al., 1993) are fitted to literature and own data for mixtures with isocyanates. The data base comprises mixtures of three different aliphatic mono-and diisocyanates with different aliphatic and aromatic solvents. The validity of the parameters is proved by predictions of other mixtures. The isocyanate group NCO is added as a new main group to the modified UNIFAC parameter matrix. There is no need for different parameter sets to describe monoisocyanates as well as diisocyanates.