Hydrodynamic, dynamo-optical and electro-optical properties of aromatic (ArIC) and aliphatic (AlIC) polyisocyanates in solution have been investigated. Experimental data show that ArIC in solution has the statistical coil conformation. The flexibility of the chain corresponds to that of typical molecules of synthetic polymers. In contrast, the optical properties of AlIC solutions in nonpolar solvents indicate that the molecular shape resembles a weakly bending rod, while the molecular chain is characterized by a high degree of intramolecular orientational order. Application of the theory of hydrodynamic and optical properties of wormlike (persistent) chains to the experimental data permits calculation of the optical anisotropy, dipole moment and persistence length of the AlIC molecules. The values obtained are similar to those for synthetic polypeptides in the α-helical conformation. The data show that the most probable conformation of AlIC in nonpolar solvents is the planar cis chain.